Handle dtypes more carefully in multi-tensor Adam

[timmoon10]

Description

One of our multi-tensor Adam kernels incorrectly downcasts an FP32 tensor to BF16 (#1863). This PR fixes that bug. I've also taken the opportunity to add more safety checks.

Closes #1863.

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

• Fix unnecessary cast in capturable Adam kernel
• Add safety checks to multi-tensor Adam functions

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[timmoon10]

/te-ci