[proto] Use the proper _transformed_types in all Transforms and eliminate unnecessary dispatching

[datumbox]

Addresses some of the remarks at #6486

This PR:

• Adds the appropriate _transformed_types types to all transforms
• Removes unnecessary dispatching within the _transform()
• Specifies where possible the return types of _transform()

[pmeier]

One question. Otherwise LGTM.

[pmeier]

LGTM!

[vfdev-5]

@datumbox CI failures are related : https://github.com/pytorch/vision/runs/8015985956?check_suite_focus=true

=================================== FAILURES ===================================
__________________ TestToImageTensor.test__transform[Image1] ___________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms.py", line 1048, in test__transform
    fn.assert_called_once_with(inpt, copy=transform.copy)
  File "/opt/hostedtoolcache/Python/3.7.13/x64/lib/python3.7/unittest/mock.py", line 888, in assert_called_once_with
    raise AssertionError(msg)
AssertionError: Expected 'to_image_tensor' to be called once. Called 0 times.
____________________ TestToImagePIL.test__transform[Image0] ____________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms.py", line 1065, in test__transform
    fn.assert_called_once_with(inpt, mode=transform.mode)
  File "/opt/hostedtoolcache/Python/3.7.13/x64/lib/python3.7/unittest/mock.py", line 888, in assert_called_once_with
    raise AssertionError(msg)
AssertionError: Expected 'to_image_pil' to be called once. Called 0 times.
________________________ test_to_image_pil[None-inpt2] _________________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms_functional.py", line 1907, in test_to_image_pil
    output = F.to_image_pil(inpt, mode=mode)
  File "/home/runner/work/vision/vision/torchvision/transforms/functional.py", line 259, in to_pil_image
    raise TypeError(f"pic should be Tensor or ndarray. Got {type(pic)}.")
TypeError: pic should be Tensor or ndarray. Got <class 'PIL.Image.Image'>.
_________________________ test_to_image_pil[RGB-inpt2] _________________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms_functional.py", line 1907, in test_to_image_pil
    output = F.to_image_pil(inpt, mode=mode)
  File "/home/runner/work/vision/vision/torchvision/transforms/functional.py", line 259, in to_pil_image
    raise TypeError(f"pic should be Tensor or ndarray. Got {type(pic)}.")
TypeError: pic should be Tensor or ndarray. Got <class 'PIL.Image.Image'>.

[vfdev-5]

@datumbox CI failures are still related to your latest commit:

=================================== FAILURES ===================================
_______________________ test_to_image_tensor[True-inpt0] _______________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms_functional.py", line 1877, in test_to_image_tensor
    output = F.to_image_tensor(inpt, copy=copy)
TypeError: to_image_tensor() got an unexpected keyword argument 'copy'
_______________________ test_to_image_tensor[True-inpt1] _______________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms_functional.py", line 1877, in test_to_image_tensor
    output = F.to_image_tensor(inpt, copy=copy)
TypeError: to_image_tensor() got an unexpected keyword argument 'copy'
_______________________ test_to_image_tensor[True-inpt2] _______________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms_functional.py", line 1877, in test_to_image_tensor
    output = F.to_image_tensor(inpt, copy=copy)
TypeError: to_image_tensor() got an unexpected keyword argument 'copy'
______________________ test_to_image_tensor[False-inpt0] _______________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms_functional.py", line 1877, in test_to_image_tensor
    output = F.to_image_tensor(inpt, copy=copy)
TypeError: to_image_tensor() got an unexpected keyword argument 'copy'
______________________ test_to_image_tensor[False-inpt1] _______________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms_functional.py", line 1877, in test_to_image_tensor
    output = F.to_image_tensor(inpt, copy=copy)
TypeError: to_image_tensor() got an unexpected keyword argument 'copy'
______________________ test_to_image_tensor[False-inpt2] _______________________
Traceback (most recent call last):
  File "/home/runner/work/vision/vision/test/test_prototype_transforms_functional.py", line 1877, in test_to_image_tensor
    output = F.to_image_tensor(inpt, copy=copy)
TypeError: to_image_tensor() got an unexpected keyword argument 'copy'

You can check that locally: pytest -vvv test/test_prototype_transforms_functional.py

[vfdev-5]

Thanks @datumbox !