Use requested mem in dot op to reduce memory usage

[bartekkuncer]

Description

This change aims to reduce memory footprint of dot operator by reusing once allocated memory buffer for future operator calls.

[mxnet-bot]

Hey @bartekkuncer , Thanks for submitting the PR
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[bartekkuncer]

@mxnet-bot run ci [centos-cpu]

[mxnet-bot]

Jenkins CI successfully triggered : [centos-cpu]

[bartekkuncer]

@mxnet-bot run ci [unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu]

[bartekkuncer]

@mxnet-bot run ci [centos-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [centos-gpu]

[bartekkuncer]

@mxnet-bot run ci [windows-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [windows-gpu]