Support automatic torchscript checkpointing.
🚀 Feature
Support exporting to TorchScript natively in the ModelCheckpoint callback. This would be a natural follow-up to #3080.
Motivation
It's common for users to rely on checkpoints (eg, usually last, but sometimes other intermediate models especially if using model ensembles or picking the best model the last k models). Frequently these models need to be served in a non-Python environment, meaning users have to convert their checkpoints to TorchScript.
With the implementation of #3080 it seems natural to add more support for TorchScript throughout the Lightning framework. In particular, we should allow users to export TorchScript models easily in checkpoints.
Pitch
We can rely on the to_torchscript method in LightningModule's to access a TorchScript version of the model when checkpointing. We would then provide an parameter in the ModelCheckpoint callback
class ModelCheckpoint(Callback):
    r"""
    Save the model after every epoch if it improves.

    After training finishes, use :attr:`best_model_path` to retrieve the path to the
    best checkpoint file and :attr:`best_model_score` to retrieve its score.

    Args:
      ...
      save_torchscript: When set, each exported model includes a Torchscript
            version. Default: ``False``.
      ...
    """       
Additional context
See #3080.