Fix crash in optimized log softmax

[mergennachin]

Fixes #13551

🐛 Problem

The optimized log_softmax kernel crashed with fatal assertions when encountering unsupported double precision dtypes:
F 00:00:00.005478 executorch:op_log_softmax.cpp:156] assert failed (false): Unhandled out dtype 7

✅ Solution

Replaced fatal ET_CHECK_MSG with graceful ET_KERNEL_CHECK error handling:

Before: Program termination on unsupported dtypesAfter: Returns error codes, allows program to continue

📝 Changes

kernels/optimized/cpu/op_log_softmax.cpp:

• Converted log_softmax_wrapper to return bool success status
• Replaced fatal assertions with graceful error returns
• Added proper error state handling via ET_KERNEL_CHECK

kernels/test/op_log_softmax_test.cpp:

• Added DoubleCase test for comprehensive double precision validation
• Implemented smart conditional testing: runs full validation on portable kernels, verifies graceful failure + skips on optimized kernels
• Uses expect_failure() for CI-friendly error state handling

🎯 Results

Kernel	Double Precision Support	Behavior	CI Status
Portable	✅ Full support	Test passes with validation	🟢 PASSED
Optimized	❌ Not supported	Graceful error + skip	🟢 SKIPPED

🛡️ Benefits

• No more crashes: Applications can handle unsupported operations gracefully
• Better testability: Error conditions can now be tested and validated
• CI compatibility: Green builds for both supported and unsupported operations
• Regression protection: Prevents future reintroduction of fatal errors

Testing: Verified on both portable and optimized kernel test suites ✅

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13953

• 📄 Preview Python docs built from this PR

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

❌ 2 New Failures

As of commit 53e3e2b with merge base 0eed262 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 08a6461d5fe1486bde00804051c4294f60e34897600866e79fac7f30ded50571 /exec failed with exit code 1

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This pull request was exported from Phabricator. Differential Revision: D81703958

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This pull request was exported from Phabricator. Differential Revision: D81703958