Rename the image decoders' OutputDtype to ImageOutputDtypeConfig

[NicolasHug]

ImageCommon.h declared facebook::torchcodec::OutputDtype {UINT8, UINT16,
AUTO} and StreamOptions.h declares facebook::torchcodec::OutputDtype
{UINT8, FLOAT32}: two different types with one fully-qualified name. They
compile today only because no translation unit includes both headers - the
image decoders are FFmpeg-free and pull in neither StreamOptions.h nor
FFMPEGCommon.h nor Frame.h - which makes it a trap for whoever first makes
those worlds touch.

The image one is the odd name rather than the odd type: it carries AUTO and
is resolved at the point of use by should_output_uint16(), so it is a
config, and it is the counterpart of OutputDtypeConfig rather than of
OutputDtype. The int64 codes crossing the op boundary are unchanged, so
_validate_output_dtype on the Python side needs nothing.

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/meta-pytorch/torchcodec/1699

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure

As of commit 89c1ff4 with merge base 2312413 ():

NEW FAILURE - The following job has failed:

• MacOS / install-and-test (3.10, 7) (gh)
  ##[error]The process '/Users/runner/miniconda3/condabin/mamba' failed with exit code 1

This comment was automatically generated by Dr. CI and updates every 15 minutes.