Silence warning in gradient checkpointing when it's False
# What does this PR do?

Currently, when loading a `bert-base-cased` model, one gets the warning 
```
UserWarning: Passing `gradient_checkpointing` to a config initialization is deprecated and will be removed in v5 Transformers. Using `model.gradient_checkpointing_enable()` instead, or if you are using the `Trainer` API, pass `gradient_checkpointing=True` in your `TrainingArguments`.
```
because it's config has the line `"gradient_checkpointing": false`.

This PR only issues the warning only when the value passed for `gradient_checkpointing` is `True`.