Add basic Mamba block

[Quentin-Anthony]

We want to add Mamba to gpt-neox:

• Add basic mamba block, without kernels, from https://github.com/state-spaces/mamba/tree/main/mamba_ssm/modules to https://github.com/EleutherAI/gpt-neox/tree/main/megatron/model
• Add mamba kernels from https://github.com/state-spaces/mamba/tree/main/mamba_ssm/ops
• Add config options for mamba
• Add assertions to gpt-neox so that parallelism schemes and other architectures are disabled when mamba is enabled in config