Add meta and OpInfo for _embedding_bag_dense_backward (#109211)
The sample inputs is a bit involved because there are a lot of
shenanigans in the derivative formula. Check comments.
This is exercised in vdd, internal test `buck2 run '@fbcode//mode/opt' fbcode//pytorch/benchmark/fb/test_gpu:run_test_gpu -- 'pytorch.benchmark.fb.test_gpu.test_gpu.TestBenchmarkFbGpu.test_train_blue_reels_vdd_v3_inductor_speedup'`
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/109211
Approved by: https://github.com/albanD, https://github.com/zou3519
