What does this PR do?

Allow users to set a custom device mesh to use with CP.

  mesh = init_device_mesh("cuda", mesh_shape=(2, 1, 4), mesh_dim_names=("ring", "ulysses", "fsdp"))
  cp_config = ContextParallelConfig(ring_degree=2, mesh=mesh)
  pipe.transformer.enable_parallelism(config=cp_config)

Fixes # (issue)
#12880

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