Added warmup parameter to early stopping cb

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What does this PR do?

This pull request introduces a new feature to the EarlyStopping callback in PyTorch Lightning, allowing users to specify the starting epoch for monitoring early stopping. Additionally, it updates imports to use the pytorch_lightning namespace and improves example documentation. Below is a summary of the most important changes:

New Feature: Starting Epoch for Early Stopping

• Added a new parameter start_from_epoch to the EarlyStopping callback, allowing users to specify the epoch from which early stopping monitoring begins. Defaults to 0. [1] [2] [3]
• Updated the _run_early_stopping_check method to skip the early stopping check if the current epoch is less than start_from_epoch.

Namespace Update

• Updated all imports from lightning.pytorch to pytorch_lightning for consistency with the latest namespace conventions. [1] [2]

Documentation Improvements

• Enhanced the example in the docstring to demonstrate how to use the new start_from_epoch parameter.

Fixes #<issue_number>

No breaking changes!

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📚 Documentation preview 📚: https://pytorch-lightning--20778.org.readthedocs.build/en/20778/