Refactor device selection: Rename to computePolicy, remove accelerated, and add fallback

[mingmingtasd]

To fix #911

Description:
This PR refactors the device selection preference API to establish a more extensible framework by replacing MLPowerPreference with MLComputePolicy in MLContextOptions.

Key changes included:

1. Renamed preference enum: MLPowerPreference has been refactored to MLComputePolicy (and the corresponding MLContextOptions dictionary member updated to computePolicy).
2. Clarified extensibility: The spec now explicitly states that the policy is designed to be extensible to accommodate future execution profiles, moving beyond just power and performance metrics.
3. Removed "accelerated" option: Removed the accelerated value to eliminate ambiguity, as performance and power semantics are better covered by other profiles.
4. Introduced "fallback" policy: Added a new "fallback" compute policy. This option prioritizes maximum compatibility over other considerations (typically running on a CPU) and is highly useful for verifying a model's numeric behavior without utilizing parallel accelerators like GPUs or NPUs.

The corresponding chromium CL is https://chromium-review.googlesource.com/c/chromium/src/+/7513189

PTAL, thanks! @huningxin

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Preview | Diff

[zolkis]

Thanks!

[fdwr]

Consider clearer naming, otherwise LGTM sir.

[mwyrzykowski]

Would like to see the references to CPU removed from the non-normative wording https://github.com/webmachinelearning/webnn/pull/923/changes#r3010808209 but otherwise no objections to the API surface changes ✅

[mingmingtasd]

I have no preference for the naming; I'll wait for the decision. For the description wording, I'm OK with not mentioning CPU to avoid being implementation-specific.

[mwyrzykowski]

✅ with the current

 "low-power",
 "fallback"

values

[anssiko]

RESOLUTION: Use MLComputePolicy "fallback" as the name for maximum compatibility preference. (PR #923)

[anssiko]

We also acknowledged the following enhancements in our discussion today:

Anssi: other enhancements from this PR, to be addressed in a separate PR:
… - "low-latency" MLComputePolicy, example use case audio processing
… - "precision" MLContextOptions, to signal chopping off low bits is not preferred for this context

[mingmingtasd]

I have no preference for the naming; I'll wait for the decision. For the description wording, I'm OK with not mentioning CPU to avoid being implementation-specific.

Thanks for your discussion! Let's keep the "fallback".

And in the latest commit, I removed the "CPU" word in the description for "fallback". @mwyrzykowski

[anssiko]

This is ready to be merged by the editors. Thank you all.

The editors can track the discussed enhancements #923 (comment) as deemed appropriate.

[fdwr]

👍

[huningxin]

LGTM, thanks @mingmingtasd !