Change default minimum_deployment_target to None and support CoreML models with no inputs

[metascroy]

This PR changes the default minimum_deployment_target in CoreML from the fixed iOS15 to None. This will automatically select the minimum deployment target required to export the model.

A warning occurs after export telling the user the deploymnet target selected, and directing them to specify a specific target if that is what they need.

This is more in line with how CoreML standalone works.

In addition, this PR allows running CoreML models with no user inputs, which requires CoreML deployment target iOS18 or higher.

This addresses:

• CoreML model with no inputs fails to load at runtime #11719
• efficient_sam fails to run on CoreML #12906

[pytorch-bot]

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❌ 2 New Failures

As of commit c5743dc with merge base 5d3550f ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255
• trunk / test-arm-cortex-m-size-test (bare_metal) / linux-job (gh)
  RuntimeError: Command docker exec -t c89e97ebaa37de119e675c9a1458ff7d88f016bfe855e06f64aef601b7edd9a3 /exec failed with exit code 1

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[metascroy]

@cymbalrush can you have a look?

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[metascroy]

@cccclai @GregoryComer let me know if you have any concerns with merging.

@cymbalrush said the changes look OK on slack

[cccclai]

I thought CoreML doesn't support version <iOS15, is it no longer true? Stamp anyway because Gyan seems good with it

[metascroy]

I thought CoreML doesn't support version <iOS15, is it no longer true? Stamp anyway because Gyan seems good with it

It doesn't support version < iOS15 if you use an mlpackage (which ET uses). This PR is about choosing higher iOS versions than iOS15 if those are required for the model. Currently it's fixed at iOS15 as the default, which could become dated.