make fill defaultdict an implementation detail

[pmeier]

Towards #7159. For v2 we added more complex handling for fill to enable filling with different values based on the type. Internally this is handled by a defaultdict:

vision/torchvision/prototype/transforms/_utils.py

Line 68 in d805aea

def _setup_fill_arg(fill: Union[FillType, Dict[Type, FillType]]) -> Dict[Type, FillTypeJIT]:

However, there is no need to leak that into a public attribute:

vision/torchvision/prototype/transforms/_geometry.py

Line 280 in d805aea

self.fill = _setup_fill_arg(fill)

This PR simply leaves parameter the user passed in the self.fill attribute and moves the defaultdict to self._fill

cc @vfdev-5 @bjuncek

[NicolasHug]

Minor comments but LGTM, thanks Philip

[pmeier]

In the transforms v1 tests we have

vision/test/test_transforms.py

Lines 1871 to 1872 in d805aea

	t = transforms.RandomRotation(0, fill=None)
	assert t.fill == 0

That will still not be supported by this change. I don't really understand the whole fill=0 vs. fill=None thing. @vfdev can you chime in here?

[vfdev-5]

I don't really understand the whole fill=0 vs. fill=None thing.

This story is certainly a tradeoff between what was supported at that moment supported values (by Pillow, pytorch) and jit, consistency etc.

For RandomRotation fill arg, it seems like the story was like that:

• Philip introduced fill=0 to the transform here: https://github.com/pytorch/vision/pull/1280/files (there was only Pillow backend at that moment)
• Then Philip changed fill=None to support older PIllow versions
• When tensor backend was introduced, fill was None as tensors could not support it
• Then there was a PR (with a bug) which added fill color support for tensor backend: Fill color support for tensor affine transforms #2904
• When the bug was discovered we changed default value to fill=0 but to keep previous behaviour we added if fill is None: fill = 0 similarly to PIL, RandomRotation and fill #3303

Today there are still few minor issues and inconcistencies for fill arg:

• Fill arg and _apply_grid_transform improvements #6517
• [BC-breaking] Aligning fill = None with fill = 0 on Transforms #6623

Also check this comment: #6623 (comment)