fix(models): Fix dtype mismatch in SwitchTransformers and TimmWrapperModel

[harshaljanjani]

What does this PR do?

The following dtype mismatch use cases were identified and fixed in this PR:

→ Switch Transformers: 7938e91fa refactored all MoE models for vLLM compatibility; in that refactor, the _cast_classifier() method was removed from SwitchTransformersTop1Router but no dtype cast was added. Casting hidden_states to classifier.weight.dtype before the linear call fixes that!
→ TimmWrapper: 6217adc6c8 changed the default dtype behavior to "auto"; in that commit, pixel_values.to(self.device, self.dtype) was regressed to pixel_values.to(self.device) dropping the dtype cast. I'm not too sure why it was dropped; but restoring it seems logical to fix the use case.
→ For more details on reproducing the bug and the output screenshots, please visit the linked issue!

cc: @Rocketknight1

Fixes #45072

CI run test coverage of this behavior (as suggested by @ydshieh) :):

SwitchTransformers:
→ test_modeling_switch_transformers.py::SwitchTransformersModelTest::test_generate_with_past_key_values
→ test_modeling_switch_transformers.py::SwitchTransformersModelTest::test_model_fp16_forward
→ test_modeling_switch_transformers.py::SwitchTransformerModelIntegrationTests::test_small_logits
TimmWrapper:
→ TimmWrapperModelTest does not have explicit bfloat16 forward pass tests; added one in this PR for complete coverage.

Repro output after the fixes (feel free to cross-check):

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[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: switch_transformers, timm_wrapper