Dedup constants in emitter

[lucylq]

Summary

Fix bug where if we have two constants with different names but the same data, the second one is not saved and results in missing tensor. (This happened in quant flows).

Test plan

python -m unittest executorch.exir.emit.test.test_emit.TestEmit.test_constant_tagged_tensor_dedup 

[pytorch-bot]

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[lucylq]

thanks for the comments @mergennachin! Updated.