[dtensor][2/N] add __repr__ to placements

[wanchaol]

Stack from ghstack (oldest at bottom):

• [WIP] prototype: op decomposition #92126
• [dtensor][6/N] change to a better/safer op registration #90735
• [dtensor][5/N] add cached propagator for TP #90734
• [dtensor][4/N] refactor dispatching logic and add propagator #90733
• [dtensor][3/N] move OpSchema and types to a separate file #90732
• -> [dtensor][2/N] add __repr__ to placements #91785
• [dtensor][1/N] add __hash__ to device_mesh and dtensor_spec #90731

This PR added repr to all placement types

[pytorch-bot]

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[XilunWu]

LGTM!