Restore is_torch_fx_available for trust_remote_code backwards compatibility

[MaybeSam05]

What does this PR do?

Restores is_torch_fx_available in transformers.utils.import_utils as a backwards-compatibility shim so that Hub models loaded with trust_remote_code=True that still import this symbol no longer raise ImportError on Transformers v5+.

Context: In v5, is_torch_fx_available was removed from import_utils. Many community models (e.g. deepseek-ai/deepseek-moe-16b-base) ship their own modeling_*.py and still do from transformers.utils.import_utils import is_torch_fx_available, which breaks at import time when using v5.

Changes:

• Re-add is_torch_fx_available() in src/transformers/utils/import_utils.py.
• The function emits a one-time DeprecationWarning (via @lru_cache), returns False if PyTorch is not available, and otherwise checks that torch.fx can be imported and returns True/False.
• Under the v5 requirement (PyTorch >= 2.4), torch.fx is always available, so behavior for existing callers is unchanged.

Fixes #44561

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Who can review?

@ArthurZucker @Rocketknight1 — this restores a symbol removed in v5 that breaks trust_remote_code models (see #44561). Optional: @Cyrilvallez for model-loading/import-utils context.

[Cyrilvallez]

cc @ArthurZucker wdyt? We could consider reintroducing, but IMO a bit pointless as again we cannot guarantee BC for remote models, and it's bound to broke in another way soon (if it's not already broken in another fashion already)