[benchmarks] Disabling gradscaler (#89741)
Disabling Gradscaler because
1) Benchmark setup runs 2 iterations of fwd-bwd. So, not useful.
2) Current setup shares grad_scaler for eager and dynamo model,
which is bad as Gradscaler has state and can adjust the scaling
factor between eager and dynamo run, making accuracy check
harder.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89741
Approved by: https://github.com/ngimel
