[ExecuTorch] Arm Ethos:Make get_compile_spec() configurable

[digantdesai]

Stack from ghstack (oldest at bottom):

• -> [ExecuTorch] Arm Ethos:Make get_compile_spec() configurable #8895
• [ExecuTorch] Arm Ethos: Add Linear operator tests #8894
• [ExecuTorch] Arm Ethos: Do not depend on torch.testing._internal  #8893

Allow users to generate different compile specs based on the Args.
Preserve the behavior.
No change in the default spec returned.

Differential Revision: D70351486

[pytorch-bot]

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This pull request was exported from Phabricator. Differential Revision: D70351486

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[AdrianLundell]

Approved with a side note that the intended use of this API is to configure the unbuilt spec, so you could achieve the same behaviour with something like:

spec = get_u55_compile_spec_unbuilt().ethosu_compile_spec(<config args>).build()

I can see that this is more convenient however.