Add entry_at(i) accessor to LoadBackendOptionsMap (#19645)

[metascroy]

Summary:

LoadBackendOptionsMap exposed get_options(backend_id), has_options(backend_id),
and size() but no way to enumerate the entries. That makes deep-copy,
debugging/pretty-printing, and any other operation that wants to iterate
over the configured backends impossible from outside the class.

Add a public EntryView struct ({backend_id, options}) plus an entry_at(i)
accessor returning a non-owning view at the given index. Header-only,
additive; existing callers are unaffected.

Reviewed By: shoumikhin

Differential Revision: D105100370

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit e9b5d80 with merge base 30c9a26 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-models-linux (mv2, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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@metascroy has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105100370.

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