test_decomp.py: Skip tests for embedding_backward bf16 (#84554)
`embedding_backward`'s decomposition is less accurate for bf16.
Currently bfloat16 is skipped in both forward and backward, but the
forward decomposition matches 1-1 with the ATen implementation so this
re-enables the test for the forwards decomposition.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84554
Approved by: https://github.com/albanD
