[BYOC][JSON] Support input nodes with multiple entries

[trevor-m]

Sometimes input nodes will have multiple data entries. The graph runtime will give a separate arg for each entry. The previous code assumed each input node would have 1 entry at index 0 which would correspond with a single arg.

Note that each custom runtime will also have the handle these types of inputs in a similar way.

Example JSON representation which would fail before but works with this PR. This assert would fail because the function has 3 args but 2 input + output nodes. See how op input has two entries for dtype and shape.

{
  "nodes": [
    {
      "op": "input", 
      "name": "tensorrt_0_i0", 
      "attrs": {
        "dtype": [
          [
            "float32", 
            "float32"
          ]
        ], 
        "shape": [
          [
            [1, 2, 6, 6], 
            [1, 3, 6, 6]
          ]
        ]
      }
    }, 
    {
      "op": "kernel", 
      "name": "concatenate", 
      "inputs": [[
          0, 
          0, 
          0], [
          0, 
          1, 
          0]], 
      "attrs": {
        "num_outputs": "1", 
        "num_inputs": "2", 
        "dtype": [
          [
            "float32"
          ]
        ], 
        "axis": [
          [
            "1"
          ]
        ], 
        "shape": [
          [
            [1, 5, 6, 6]
          ]
        ]
      }
    }
  ], 
  "arg_nodes": [0], 
  "heads": [[
      1, 
      0, 
      0]], 
  "node_row_ptr": [0, 2, 3]
}

[trevor-m]

@zhiics @comaniac Could you please take a look? Thanks!

[comaniac]

LGTM. Just a nit.

[comaniac]

By doing so, input_var_idx_ is not keeping index but entry ID. I'd suggest renaming it to be clearer.

[zhiics]

Thanks @trevor-m @comaniac