[Torch] Restore class-aware NMS for detection models by graph rewrite (…

…apache#7154)

* add a pattern to rewrite nms to batched nms

* update object detection test to add rewrite

* updated tutorial

* add doc

* fixed coord_start

* test fixed by setting force_surpress=False

* revert tutorial change

* add some comment to explain the pattern

* update NMS pattern following frontend change

python/tvm/relay/frontend/pytorch.py

@@ -1866,26 +1866,26 @@ def nms(self, inputs, input_types):
         scores = inputs[1]
         iou_threshold = inputs[2]
 
-        num_boxes = _op.shape_of(scores)
-
         # TVM NMS assumes score > 0
         scores = scores - _op.min(scores) + _op.const(1.0)
+
+        num_boxes = _op.shape_of(scores)
+        # PyTorch NMS doesn't have score_threshold, so no need to run get_valid_count
+        indices = _op.transform.arange(_op.squeeze(num_boxes), dtype="int32")
+        indices = _op.expand_dims(indices, 0, 1)
+
         # Generate data with shape (1, num_anchors, 5)
         scores = AttrCvt(op_name="expand_dims", extras={"axis": -1, "num_newaxis": 1})([scores], {})
         data = _op.concatenate([scores, boxes], -1)
         data = _op.expand_dims(data, 0, 1)
-        # PyTorch NMS doesn't have score_threshold, so no need to run get_valid_count
-        indices = _op.transform.arange(_op.squeeze(num_boxes), dtype="int32")
-        indices = _op.expand_dims(indices, 0, 1)
-        ct = num_boxes
 
         # Perform Non-Maximum Suppression,
         # PyTorch NMS doesn't have parameter top_k and max_output_size
         score_index = 0
         top_k = max_out_size = -1
         nms_ret = get_relay_op("non_max_suppression")(
             data=data,
-            valid_count=ct,
+            valid_count=num_boxes,
             indices=indices,
             max_output_size=max_out_size,
             iou_threshold=iou_threshold,

python/tvm/relay/frontend/pytorch_utils.py

@@ -14,8 +14,17 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
-# pylint: disable=import-outside-toplevel
+# pylint: disable=import-outside-toplevel, unused-argument, invalid-name
 """ Common utilities used by PyTorch frontend """
+from .. import op
+from ..dataflow_pattern import (
+    is_constant,
+    is_op,
+    rewrite,
+    is_tuple,
+    wildcard,
+    DFPatternCallback,
+)
 
 
 def is_version_greater_than(ver):
@@ -25,3 +34,145 @@ def is_version_greater_than(ver):
     return "".join(re.findall(r"(\d+\.)(\d+\.)(\d)", torch.__version__)[0]) > "".join(
         re.findall(r"(\d+\.)(\d+\.)(\d)", ver)[0]
     )
+
+
+def batched_nms_pattern(boxes, scores, idxs, iou_threshold, num_boxes, indices):
+    """A pattern to detect batched_nms function in torchvision
+
+    The inputs to this function, boxes, scores, idxs, iou_threshold are wildcard
+    patterns which can be used later in the rewriting to extract matched Relay fragments.
+
+    We want to detect the following PyTorch code snippet:
+
+    def batched_nms(boxes, scores, idxs, iou_threshold):
+        max_coordinate = boxes.max()
+        offsets = idxs.to(boxes) * (max_coordinate + torch.tensor(1).to(boxes))
+        boxes_for_nms = boxes + offsets[:, None]
+        keep = nms(boxes_for_nms, scores, iou_threshold)
+        return keep
+
+    Here is how PyTorch frontend lowers above PyTorch code. For simplicity, Relay ops for
+    dealing with dynamic strided_slice are omitted. %num_boxes, %indices are complex
+    expressions, but since we can use the wildcard part for them, we do not need to construct
+    their patterns.
+
+    %2 = expand_dims(%scores, axis=-1);
+    %3 = cast(%idxs, dtype="float32");
+    %4 = max(%boxes);
+    %5 = add(%4, 1f);
+    %6 = multiply(%3, %5);
+    %7 = strided_slice(%6, begin=[0], end=[4507], strides=[1]);
+    %8 = expand_dims(%7, axis=1);
+    %9 = add(%boxes, %8);
+    %10 = (%2, %9);
+    %11 = concatenate(%10, axis=-1);
+    %12 = expand_dims(%11, axis=0);
+    ...
+    ...
+    %17 = vision.non_max_suppression(%12, %num_boxes, %indices, -1, 0.7f, ...);
+
+    """
+    one = is_constant()
+    zero = is_constant()
+
+    # Equivelent PyTorch code from above snippet
+    # offsets = idxs.to(boxes) * (max_coordinate + torch.tensor(1).to(boxes))
+    cast = is_op("cast")(idxs)
+    mx = is_op("max")(boxes)
+    add = is_op("add")(mx, one)
+    mul = is_op("multiply")(cast, add)
+
+    # The following doesn't appear in the above Relay snippet. It is required for dynamic
+    # stride_slice handling
+    cast_like = is_op("cast_like")(zero, is_constant())
+    less = is_op("less")(is_constant(), cast_like)
+    shape_of = is_op("shape_of")(mul)
+    cast_like = is_op("cast_like")(shape_of, is_constant())
+    add = is_op("add")(is_constant(), cast_like)
+    where = is_op("where")(less, add, is_constant())
+    shape_of = is_op("shape_of")(mul)
+    cast = is_op("cast")(shape_of)
+
+    # This corresponds to offsets[:, None], where offsets is the result of multiplication
+    dyn_strided_slice = is_op("dyn.strided_slice")(mul, where, cast, is_constant())
+
+    # Add offsets to the boxes
+    expand_dims = is_op("expand_dims")(dyn_strided_slice)
+    add = is_op("add")(boxes, expand_dims)
+
+    # The rest of patterns correspond to the PyTorch frontend conversion
+    # function for torchvision::nms
+    score_expand_dims = is_op("expand_dims")(scores)
+    tup = is_tuple([score_expand_dims, add])
+    concat = is_op("concatenate")(tup)
+    data = is_op("expand_dims")(concat)
+
+    return is_op("vision.non_max_suppression")(
+        data, num_boxes, indices, is_constant(), iou_threshold
+    )
+
+
+class NMSRewrite(DFPatternCallback):
+    """A callback to rewrite nms and restore batched nms"""
+
+    def __init__(self):
+        super().__init__()
+        # exprs to extract
+        self.boxes = wildcard()
+        self.scores = wildcard()
+        self.idxs = wildcard()
+        self.iou_threshold = wildcard()
+        self.num_boxes = wildcard()
+        self.indices = wildcard()
+
+        self.pattern = batched_nms_pattern(
+            self.boxes,
+            self.scores,
+            self.idxs,
+            self.iou_threshold,
+            self.num_boxes,
+            self.indices,
+        )
+
+    def convert_batched_nms(self, boxes, scores, idxs, iou_thres, num_boxes, indices):
+        """Restore class-aware NMS using extracted class indices"""
+        scores = op.expand_dims(scores, axis=-1, num_newaxis=1)
+        idxs = op.expand_dims(idxs, axis=-1, num_newaxis=1)
+        idxs = op.cast(idxs, "float32")
+        data = op.concatenate([idxs, scores, boxes], -1)
+        data = op.expand_dims(data, 0, 1)
+
+        top_k = max_out_size = -1
+        out = op.vision.non_max_suppression(
+            data=data,
+            valid_count=num_boxes,
+            indices=indices,
+            max_output_size=max_out_size,
+            iou_threshold=iou_thres,
+            force_suppress=False,
+            top_k=top_k,
+            coord_start=2,
+            score_index=1,
+            id_index=0,
+            return_indices=True,
+            invalid_to_bottom=False,
+        )
+        return out.tuple_value
+
+    def callback(self, pre, post, node_map):
+        boxes = node_map[self.boxes][0]
+        scores = node_map[self.scores][0]
+        idxs = node_map[self.idxs][0]
+        iou_thres = node_map[self.iou_threshold][0]
+        num_boxes = node_map[self.num_boxes][0]
+        indices = node_map[self.indices][0]
+        return self.convert_batched_nms(boxes, scores, idxs, iou_thres, num_boxes, indices)
+
+
+def rewrite_nms_to_batched_nms(mod):
+    """Rewrite the input graph to replace non maximum surpression
+    in torchvision that does not take class id into account with the one
+    that avoids IOU tests between different classes.
+    """
+    mod["main"] = rewrite(NMSRewrite(), mod["main"])
+    return mod

tests/python/frontend/pytorch/test_object_detection.py

@@ -26,6 +26,7 @@
 import tvm.testing
 from tvm import relay
 from tvm.runtime.vm import VirtualMachine
+from tvm.relay.frontend.pytorch_utils import rewrite_nms_to_batched_nms
 from tvm.contrib.download import download
 
 
@@ -108,15 +109,17 @@ def test_detection_models():
     with torch.no_grad():
         pt_res = scripted_model(data)
 
-    for target in ["llvm", "cuda"]:
+    def compile_and_run_vm(mod, params, data_np, target):
         with tvm.transform.PassContext(opt_level=3):
             vm_exec = relay.vm.compile(mod, target=target, params=params)
 
         ctx = tvm.context(target, 0)
         vm = VirtualMachine(vm_exec, ctx)
-
         vm.set_input("main", **{input_name: data_np})
-        tvm_res = vm.run()
+        return vm.run()
+
+    for target in ["cuda", "llvm"]:
+        tvm_res = compile_and_run_vm(mod, params, data_np, target)
 
         # Bounding boxes
         tvm.testing.assert_allclose(
@@ -132,3 +135,14 @@ def test_detection_models():
         score_threshold = 0.9
         print("Num boxes:", pt_res[0].cpu().numpy().shape[0])
         print("Num valid boxes:", np.sum(pt_res[1].cpu().numpy() >= score_threshold))
+
+    before = mod["main"]
+    mod = rewrite_nms_to_batched_nms(mod)
+    after = mod["main"]
+    assert not tvm.ir.structural_equal(after, before)
+
+    tvm_res_after_rewrite = compile_and_run_vm(mod, params, data_np, "llvm")
+
+    # Results should be equivalent after rewriting
+    for res1, res2 in zip(tvm_res, tvm_res_after_rewrite):
+        tvm.testing.assert_allclose(res1.asnumpy(), res2.asnumpy())