Allow decoding functions to accept the mode parameter as a string

[NicolasHug]

Closes https://github.com/pytorch/vision/issues/8625

the mode parameter of decoding functions can now be passed as a string, instead of just as an enum. Also added RGBA as an alias of RGB_ALPHA.

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8627

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:white_check_mark: You can merge normally! (1 Unrelated Failure)

As of commit 005f094b0159bf32f8ce4aa7ec4fde3d2632c943 with merge base d0ebeb55573820df89fa24f6418b9e624683932d ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

* [Tests / unittests-windows (3.9, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job](https://hud.pytorch.org/pr/pytorch/vision/8627#29625825837) ([gh](https://github.com/pytorch/vision/actions/runs/10686953132/job/29625825837)) ([trunk failure](https://hud.pytorch.org/pytorch/vision/commit/d0ebeb55573820df89fa24f6418b9e624683932d#29626014507)) `AttributeError: '_OpNamespace' 'image' object has no attribute '_jpeg_version'`

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