CLI for inspecting checkpoints

@awaelchli is already working on this.

Since Mar 8, 2021.

## 🚀 Feature

A quick way to inspect checkpoint files.

### Motivation

We don't know what's inside a checkpoint. Loading one and checking the contents requires us to write a python script.
A command line tool could help us load and inspect these checkpoint files conveniently. Useful for debugging too.

### Pitch

My implementation is [here](https://github.com/awaelchli/pytorch-lightning-snippets/blob/master/checkpoint/peek.py).
[Docs](https://github.com/awaelchli/pytorch-lightning-snippets#inspect-checkpoint-files)

```
Usage:

> peek path/to/checkpoint.ckpt
displays all top-level variables in a checkpoint file

> peek -i path/to/checkpoint.ckpt
interactive mode, loads the contents in a python interactive shell and let's you modify the contents
```


Let me know if you want it

cc @borda @tchaton @justusschock @awaelchli @ananthsub @ninginthecloud @rohitgr7