handle get_input_embeddings() on models like gemma3 gracefully
Author: winglian
Pull request: yes

# What does this PR do?

#41993 breaks calls to `model.enable_input_require_grads()`

```
stderr: [rank1]:   File "/home/wing/axolotl-dev/src/axolotl/loaders/adapter.py", line 188, in load_adapter
stderr: [rank1]:     model.enable_input_require_grads()
stderr: [rank1]:   File "/home/wing/.venvs/axolotl/lib/python3.11/site-packages/transformers/modeling_utils.py", line 1986, in enable_input_require_grads
stderr: [rank1]:     input_embeddings = module.get_input_embeddings()          
stderr: [rank1]:                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                                        
stderr: [rank1]:   File "/home/wing/.venvs/axolotl/lib/python3.11/site-packages/transformers/modeling_utils.py", line 993, in get_input_embeddings
stderr: [rank1]:     raise NotImplementedError(
stderr: [rank1]: NotImplementedError: `get_input_embeddings` not auto‑handled for SiglipVisionTransformer; please override in the subclass.
```
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