Allow more then 1 NLP node
Current __init__ to 1 will not allow expansions to over 1 node when using a https://pytorch-lightning.readthedocs.io/en/latest/clouds/cluster.html#general-purpose-cluster
Changing to None allows for replacement.

with num_nodes: int = 1 and 2 nodes requested 
Node 1 
[NeMo I 2021-07-29 00:29:42 nlp_overrides:56] 
     init_ddp_connection global_rank 0 world_size 1
initializing ddp: GLOBAL_RANK: 0, MEMBER: 1/1
Added key: store_based_barrier_key:1 to store for rank: 0
Rank 0: Completed store-based barrier for 1 nodes.
[NeMo I 2021-07-29 00:29:42 nlp_overrides:65] 
     init_ddp_connection global_rank 0 world_size 1

Node 2 
[NeMo I 2021-07-29 00:29:42 nlp_overrides:56] 
     init_ddp_connection global_rank 1 world_size 1
initializing ddp: GLOBAL_RANK: 1, MEMBER: 2/1
Added key: store_based_barrier_key:1 to store for rank: 1

with num_nodes: int = None,
Node 1 
[NeMo I 2021-07-29 00:54:36 nlp_overrides:56] 
     init_ddp_connection global_rank 0 world_size 2
initializing ddp: GLOBAL_RANK: 0, MEMBER: 1/2
Added key: store_based_barrier_key:1 to store for rank: 0
Rank 0: Completed store-based barrier for 2 nodes.
[NeMo I 2021-07-29 00:54:37 nlp_overrides:65] 
     init_ddp_connection global_rank 0 world_size 2

Node 2 
[NeMo I 2021-07-29 00:54:37 nlp_overrides:56] 
     init_ddp_connection global_rank 1 world_size 2
initializing ddp: GLOBAL_RANK: 1, MEMBER: 2/2
Added key: store_based_barrier_key:1 to store for rank: 1
Rank 1: Completed store-based barrier for 2 nodes.
[NeMo I 2021-07-29 00:54:37 nlp_overrides:65] 
     init_ddp_connection global_rank 1 world_size 2
