Minor issues on `PatchEmbed` code

https://github.com/facebookresearch/SlowFast/blob/5b5d9ecb15a54188943af0cbf5f7c420d8409018/slowfast/models/stem_helper.py#L325

in this line if `keep_spatial` is `False`, module flattens and transposes output tensor and return it with its shape. However according to the original code `return x.flatten(2).transpose(1, 2), x.shape` won't return its changed shape. It will always returns original output `x`'s shape. If the purpose is returning changed shape, then the code should be like
```python
x = x.flatten(2).transpose(1, 2)
return x, x.shape
```