Sm trainer smp init fix
# What does this PR do?

Fixes `SageMakerTrainer` `smp.init` for `smp 1.3`. It also removes the `is_smdistributed_available` for are more robust `is_sagemaker_model_parallel_available`. 

LGTM! I ran one training job successfully with these changes. <|||||>I tested it with `pytorch1.7.1`
`564829616587.dkr.ecr.us-east-1.amazonaws.com/huggingface-pytorch-training:1.7.1-transformers4.4.0-py36-gpu-cu110-ubuntu18.04` 
and `pytorch1.6` 
`763104351884.dkr.ecr.us-east-1.amazonaws.com/huggingface-pytorch-training:1.6.0-transformers4.4.2-gpu-py36-cu110-ubuntu18.04`
