Add 'ml.p5.48xlarge' as a supported instance for SM_EFA_NCCL_INSTANCES.

**Describe the feature you'd like**
Add "ml.p5.48xlarge" to the list of SM_EFA_NCCL_INSTANCES. This means SageMaker Training Toolkit sets FI_PROVIDER=efa automatically for use in training jobs to ensure EFA is used.

**How would this feature be used? Please describe.**
Automatically setting EFA for use in SageMaker Training Jobs that leverage the SageMaker Training Toolkit

**Describe alternatives you've considered**
Unsure. Currently you would manually set the environment variable.

**Additional context**
NA

Search the repository for SM_EFA_NCCL_INSTANCES and inspect the surrounding SageMaker Training Toolkit configuration. Confirm how the list controls automatic FI_PROVIDER=efa behavior, then verify that ml.p5.48xlarge is included and that the existing tests or checks pass.

Written by the indexing model from the issue text.