expose some prototype transforms utils

[pmeier]

In order to make it easy to subclass the v2 transforms, we also need to expose some utilities for public usage. This PR adds a new module torchvision.prototype.transforms.utils for that. Everything else stays in _utils as before.

For completeness, we also discussed dynamically create this module. This can be achieved by adding the following to __init__.py

def _load_utils():
    import sys
    import types

    from torchvision.prototype.transforms import _utils

    utils = types.ModuleType(f"{__package__}.utils")
    utils.__package__ = __package__
    for attr in ["has_any", "has_all"]:  # TODO: expand this to everything we want to expose
        utils.__dict__[attr] = getattr(_utils, attr)
    sys.modules[utils.__name__] = utils

    return utils


utils = _load_utils()

del _load_utils

Now, one can do

>>> from torchvision.prototype.transforms import utils
>>> utils.has_any
<function has_any at 0x7f92516060e0>
>>> from torchvision.prototype.transforms.utils import has_any
>>> has_any
<function has_any at 0x7f92516060e0>

This would save us the split between the two modules, but has some issues as well:

1. This is a fairly complex solution to an otherwise simple problem
2. Due to the dynamic creation, imports such as from torchvision.prototype.transforms.utils import has_any work perfectly fine, but are not picked up by static analyzers like mypy or IDE's

cc @vfdev-5 @datumbox @bjuncek

[pmeier]

This is the only thing we need to deal with before we move forward.

[datumbox]

I had a look on the usage of _isinstance in our codebase and feel that it's useful enough to be a public utility. WDYT?

We obviously can't remove the underscore, so we need a new name. Perhaps check_type()?

[pmeier]

Yes, I would expose it as well. +1 for check_type. @vfdev-5 Do you have an opinion here?

[datumbox]

When we merge this, we should update #6721 and #6534 to avoid breaking their code again.

[datumbox]

@pmeier LGTM, thanks!

@vfdev-5 We are having issues with the flaky bbox resize tests. Could you please prioritise the fix?

[pmeier]

We are having issues with the flaky bbox resize tests. Could you please prioritise the fix?

See #6995.