Add support to modify nemo cache directory
# Changelog
- Adds support to modify the nemo cache directory using an environment variable

# Usage

```sh
NEMO_CACHE_DIR="~/nemo_cache_dir/" python nemo_example_script.py
```

Closes https://github.com/NVIDIA/NeMo/issues/3185, https://github.com/NVIDIA/NeMo/issues/2025

Signed-off-by: smajumdar <titu1994@gmail.com> 