Count Sketch Backward, CUDA Memcheck failures

[anirudh2290]

Description

There are cuda memcheck failures for count sketch backward that need to be addressed. There is an invalid read of size 4 bytes that need to be fixed for count sketch backward.
I fixed by adding an index bound on nthreads but that causes a new issue.
The backward pass of this OP needs to be fixed.
This was again encountered during #15118

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[mxnet-label-bot]

Hey, this is the MXNet Label Bot.
Thank you for submitting the issue! I will try and suggest some labels so that the appropriate MXNet community members can help resolve it.
Here are my recommended labels: Cuda, Bug

[leleamol]

@mxnet-label-bot add [Cuda, Bug]