fix OOM error during resource allocation

[szha]

Description

fix OOM error during resource allocation for dropout state

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• Code is well-documented:
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• alloc and direct free for cudnn dropout state space

Comments

• the exact situation for reproducing is hard to achieve so I didn't add a test. the fix follows https://github.com/apache/incubator-mxnet/pull/12804/files#diff-45a531b6c3e468efe09d3a66d99429b1R1016

[wkcn]

Thank you for the fix!
LGTM

[wkcn]

Is there any other state_space which need to reserve memory?
Could we add a function to reserve memory for the memory pool?

[szha]

@wkcn I think most other cases are already handled manually (e.g. cudnn CNN/RNN). It might be worth a refactor for this though it's not urgent.

[szha]

And thanks for the quick review!