Increase slow grad check timeout (#89079)
Now that periodic jobs are run under `mem_leak_check` mode with parallelization turning off. It's very easy for `linux-bionic-cuda11.6-py3-gcc7-slow-gradcheck / test` to timeout because one of the shards is very close to the 4h mark.
* https://hud.pytorch.org/pytorch/pytorch/commit/2452e3f99a072760fc46d3f9025aaa37ca7ea2ab
* https://hud.pytorch.org/pytorch/pytorch/commit/35e668b5ced25e735b6e523d557ed7fd60267914
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89079
Approved by: https://github.com/clee2000
