[deepcam] Update readme for new LR warmup schedule requirements

[sparticlesteve]

The deepcam readme still describes the dependency on an external package for the LR warmup scheduler: https://github.com/mlcommons/hpc/blob/main/deepcam/README.md#before-you-run

My understanding is this is no longer needed because the code was updated to have its own implementation. @azrael417 can you confirm and update the readme?