'Cache only has 0 layers' during generation after upgrading Transformers from 4.49 to 4.50
Author: edwardlee4948
Pull request: no
Labels: name: bug

name: Cache

### System Info

After updating transformers from version 4.49.0 to 4.50.0, running .generate() on a PEFT model throws a KeyError related to cache layers:

`  File "//.venv/lib/python3.12/site-packages/peft/peft_model.py", line 1874, in generate
    outputs = self.base_model.generate(*args, **kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "//.venv/lib/python3.12/site-packages/unsloth/models/llama.py", line 1579, in unsloth_fast_generate
    output = self._old_generate(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "//.venv/lib/python3.12/site-packages/torch/utils/_contextlib.py", line 116, in decorate_context
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "//.venv/lib/python3.12/site-packages/transformers/generation/utils.py", line 2326, in generate
    result = self._sample(
             ^^^^^^^^^^^^^
  File "//.venv/lib/python3.12/site-packages/transformers/generation/utils.py", line 3286, in _sample
    outputs = self(**model_inputs, return_dict=True)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "//.venv/lib/python3.12/site-packages/torch/nn/modules/module.py", line 1739, in _wrapped_call_impl
    return self._call_impl(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "//.venv/lib/python3.12/site-packages/torch/nn/modules/module.py", line 1750, in _call_impl
    return forward_call(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "//.venv/lib/python3.12/site-packages/unsloth/models/llama.py", line 1026, in _CausalLM_fast_forward
    outputs = fast_forward_inference(
              ^^^^^^^^^^^^^^^^^^^^^^^
  File "//.venv/lib/python3.12/site-packages/unsloth/models/llama.py", line 938, in LlamaModel_fast_forward_inference
    seq_len = past_key_values[0][0].shape[-2]
              ~~~~~~~~~~~~~~~^^^
  File "//.venv/lib/python3.12/site-packages/transformers/cache_utils.py", line 387, in __getitem__
    raise KeyError(f"Cache only has {len(self)} layers, attempted to access layer with index {layer_idx}")
KeyError: 'Cache only has 0 layers, attempted to access layer with index 0'`


Let me know if you want me to cross-check for any known changes in 4.50 related to cache handling.

### Who can help?

_No response_

### Information

- [ ] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

To reproduce the KeyError: Cache only has 0 layers error using Unsloth with transformers==4.50.0, here’s a minimal working example that triggers the issue during .generate()

### Expected behavior

It should run .generate() without raising a KeyError, as it did under transformers==4.49.0.

Comments:
Hey! We would need a minimal reproducer which doesn't rely on Unsloth so we can help you 🤗 

cc @gante 

Possibly related to #37014 

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.