Add Apple Metal (MPS) GPU support

[kharaldsson]

• Creates a conda environment sufficient to support training using mps compute on an Apple M[>=1] machine
• Updates the names of existing conda environments to reflect cuda vs mps requirements

Note: While pytorch recently added support for mixed precision training using an mps backend, huggingface has yet to update the accelerate library accordingly (see, I think, accelerate autocast on mps device #1396)