Updated fill arg typehint for affine, perspective and elastic ops

[vfdev-5]

Follow-up to #6594

[datumbox]

Overall looks good. A few questions/suggestions but nothing major. Let me know your thoughts.

[datumbox]

I know this kernel is private on stable and I agree with the change to ensure consistency. Any BC breakages that can creep upwards due to the change on the default value?

[vfdev-5]

pad_image_tensor is low-level new API so I think there is no BC to keep. In any case fill=None will be transformed to fill=0 inside

vision/torchvision/transforms/functional_tensor.py

Lines 381 to 382 in c0911e3

	if fill is None:
	fill = 0

[datumbox]

I just wonder if we could do something like:

Suggested change

	if fill is None:
	# This is a JIT workaround
	return _pad_with_scalar_fill(img, padding, fill=None, padding_mode=padding_mode)
	elif isinstance(fill, (int, float)) or len(fill) == 1:
	fill_number = fill[0] if isinstance(fill, list) else fill
	return _pad_with_scalar_fill(img, padding, fill=fill_number, padding_mode=padding_mode)
	if fill is None or isinstance(fill, (int, float)) or len(fill) == 1:
	if fill is None:
	fill = 0
	fill_number = fill[0] if isinstance(fill, list) else fill
	return _pad_with_scalar_fill(img, padding, fill=fill_number, padding_mode=padding_mode)

To avoid the duplicate dispatch to scalar. I assume that fill-None ends up becoming 0?

[vfdev-5]

Functional tensor is already doing

vision/torchvision/transforms/functional_tensor.py

Lines 381 to 382 in c0911e3

	if fill is None:
	fill = 0

[vfdev-5]

I was thinking that below code is valid, but JIT does not agree

    if fill is None or isinstance(fill, (int, float)) or len(fill) == 1:
        fill_number = fill[0] if isinstance(fill, list) else fill

[datumbox]

If what I proposed above makes sense, you can avoid the optional here:

Suggested change

	fill: Optional[Union[int, float]] = None,
	fill: Union[int, float],

At any case I don't think we should pass a default value here None. We should always receive it as a positional arg, similar to what we do for _pad_with_vector_fill

[vfdev-5]

I can change it to fill: Union[int, float, None] and remove default value.

[datumbox]

@vfdev-5 The work of making F JIT-scriptable (see #6584) depends on this PR. We should consider replacing Sequences for Lists and updating the _convert_fill_arg() to be JIT scriptable.

[pmeier]

Nothing stands out, so LGTM if CI is green.