Add option for specifying wandb save_dir from config
Signed-off-by: Shantanu Acharya <shantanua@nvidia.com>

# What does this PR do ?

A user can now specify the name of the directory to save the wandb logs by setting the value `exp_manager.wandb_logger_kwargs.save_dir` in the config.

**Collection**: [utils]

# Changelog 

Files changed: `nemo/utils/exp_manager.py` (line: 670-671)

```
if wandb_kwargs.get('save_dir', None) is None:
    wandb_kwargs['save_dir'] = exp_dir
os.makedirs(wandb_kwargs['save_dir'], exist_ok=True)
wandb_logger = WandbLogger(version=version, **wandb_kwargs)
```

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

## Who can review?

Anyone in NeMo
