Fix model infrastructure edge cases

[taivu1998]

Summary

Fixes #13655.

This PR addresses several small model-infrastructure edge cases that can surface as runtime failures or state leaks:

• restores AutoModel.config_name after model-index probing, config fallback, and exceptions
• supports local PathLike model paths when building _diffusers_load_id
• clears CacheMixin.cache_context() hook state even when the wrapped block raises
• keeps FIR up/downsampling kernels on the input device and dtype for low-precision tensors
• fixes XLA availability checks to call the helper and raise real ImportErrors
• fixes enable_parallelism() to reject unavailable or uninitialized distributed setups
• preserves unused kwargs when FlaxModelMixin.from_pretrained() receives an explicit config
• defaults 1D sin/cos positional embedding calculations to torch.float32

Root Cause

The failures come from a few independent guard/state issues:

• AutoModel.from_pretrained() temporarily mutated the class-level config_name without guaranteed restoration and joined load-id parts without stringifying path objects.
• cache_context() reset hook context only on normal exit, so exceptions could leave stateful hooks bound to a stale context.
• FIR helpers rebuilt kernels as float32 tensors after receiving low-precision inputs, which could mismatch bfloat16 or other low-precision activations/weights.
• A few availability checks tested function objects instead of calling them, raised strings instead of exceptions, or used and where or was required for guard logic.
• The Flax explicit-config path skipped initialization of unused_kwargs.

Validation

Ran focused and broader checks locally:

• pytest tests/models/test_modeling_utils.py -q
• pytest tests/models/test_modeling_flax_utils.py -q
• pytest tests/hooks/test_hooks.py -q -k cache_context_clears_stateful_hook_context_after_exception
• pytest tests/models/test_attention_processor.py -q -k 'xla_flash_attention or is_torch_xla_version'
• pytest tests/models/test_layers_utils.py -q -k 'sincos_pos_embed or fir_bfloat16 or fir_upsample_with_conv_bfloat16 or fir_downsample_with_conv_bfloat16'
• pytest tests/models/test_models_auto.py -q -k 'local_model_index_pathlike or local_config_fallback or load_config_exception'
• pytest tests/hooks/test_hooks.py -q
• pytest tests/models/test_layers_utils.py -q
• uvx ruff check ...
• uvx ruff format --check ...
• git diff --check