fix: Dont explicitly source AWS plugin on Polaris

[saforem2]

Copilot Summary

This pull request makes minor adjustments to the ALCF/helpers.sh script, primarily focusing on changing the default data type and commenting out plugin setup for AWS NCCL OFI on Polaris.

Configuration updates:

• Changed the default value of the DTYPE environment variable from fp16 to bf16 in the setParams() function, making bfloat16 the default data type.

Infrastructure setup:

• Commented out the sourcing of the AWS NCCL OFI Plugin script for Polaris, so it is no longer set up by default.

Other:

• Removed the loss_scale parameter from the bfloat16 section in the generated DeepSpeed config.