NMT checkpoint averaging
This PR adds convenience scripts to checkpoint average any `.nemo` file (regardless of the class).
Python script is heavily based on code by @MaximumEntropy and @titu1994.

# Usage

The script `checkpoint_averaging.py` builds a `.nemo` file with average weights over multiple `.ckpt` files (assumes `.ckpt` files in same folder as `.nemo` file).
The input is a list of folders that contains a .nemo file (or a list of .nemo files).
The output of the script (per input folder / `.nemo` file) is a file named `*-averaged.nemo` where `*.nemo` is the original `.nemo` file.

# Example

Usage example for building `*-averaged.nemo` for a given `.nemo` file:

```
NeMo/scripts/checkpoint_averaging/checkpoint_averaging.py my_model.nemo
```

Usage example for building *-averaged.nemo files for all results in sub-directories under current path:

```
find . -name '*.nemo' | grep -v -- "-averaged.nemo" | xargs NeMo/scripts/checkpoint_averaging/checkpoint_averaging.py
```