[Whisper] Fix gradient checkpointing
Author: sanchit-gandhi
Pull request: yes

# What does this PR do?

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Fixes #19537

Sanity check:
```python
from transformers import WhisperFeatureExtractor, WhisperConfig, WhisperForConditionalGeneration
import numpy as np

feature_extractor = WhisperFeatureExtractor()
config = WhisperConfig()
model_encoder = WhisperForConditionalGeneration(config).model.encoder

# enable checkpointing
model_encoder.gradient_checkpointing_enable()

# create dummy audio input
sample = {"array": np.ones(1000), "sampling_rate": 16000}

# pre-process audio input
inputs = feature_extractor(sample["array"], sampling_rate=sample["sampling_rate"], return_tensors="pt").input_features

# forward pass
outputs = model_encoder(inputs)
```

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Thanks!