[hotfix] fix stable diffusion inference bug. (hpcaitech#5289)

* Update train_ddp.yaml

delete  "strategy" to fix DDP config loading bug in "main.py"

* Update train_ddp.yaml

fix inference with scripts/txt2img.py config file load bug.

* Update README.md

add pretrain model test code.

examples/images/diffusion/README.md

@@ -202,6 +202,8 @@ python main.py --logdir /tmp/ -t -b configs/Teyvat/train_colossalai_teyvat.yaml
 ```
 
 ## Inference
+if you want to test with pretrain model,as bellow:
+python scripts/txt2img.py --prompt "a photograph of an astronaut riding a horse" --plms    --outdir ./output     --ckpt 512-base-ema.ckpt     --config configs/train_ddp.yaml
 
 You can get your training last.ckpt and train config.yaml in your `--logdir`, and run by
 ```

examples/images/diffusion/configs/train_ddp.yaml

@@ -1,5 +1,6 @@
 model:
   base_learning_rate: 1.0e-4
+  target: ldm.models.diffusion.ddpm.LatentDiffusion
   params:
     parameterization: "v"
     linear_start: 0.00085
@@ -86,8 +87,6 @@ lightning:
     max_epochs: 2
     precision: 16
     auto_select_gpus: False
-    strategy:
-      find_unused_parameters: False
     log_every_n_steps: 2
 #    max_steps: 6o
     logger: True