Update deprecated pytorch_lightning flags in NeMo
# What does this PR do?

Please have one of the following:

- Update pytorch_lightning deprecated variables, namely:

> - accelerator to the trainer now takes the type of accelerator device than distributed type 
> - distributed type is handled by strategy flag
> - previously: accelerator='ddp' now accelerator='gpu' strategy='ddp' 
> - trainer flag checkpoint_callback is renamed to enable_checkpointing
> - ModelCheckpoint argument evert_n_val_epochs renamed to every_n_epochs 

Reference: https://pytorch-lightning.readthedocs.io/en/1.5.9/common/trainer.html#trainer-flags 

## Before your PR is "Ready for review"
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [x] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?


If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?
@titu1994 