Potential issue with exp_manager checkpoint loading
**Describe the bug**

If need to resume checkpoints from `exp_manager.explicit_log_dir`, NeMo exp_manager will check the folder for checkpoints with `end.ckpt` or `last.ckpt` suffix. However, sometimes if one of the data_parallel_rank 0 failed to save the checkpoint or there are multiple `last.ckpt` in the same folder, the workers will fail to load the correct checkpoints. 

**Expected behavior**

In exp_manager, only the global rank 0 will check which last ckpt to load and then broadcast the ckpt path to other workers. 

**Environment overview (please complete the following information)**

 - Environment location: [Bare-metal, Docker, Cloud(specify cloud provider - AWS, Azure, GCP, Collab)]
 - Method of NeMo install: [pip install or from source]. Please specify exact commands you used to install.
 - If method of install is [Docker], provide `docker pull` & `docker run` commands used

**Environment details**

If NVIDIA docker image is used you don't need to specify these.
Otherwise, please provide:
- OS version
- PyTorch version
- Python version

**Additional context**

Add any other context about the problem here.
Example: GPU model
