[js/web/training] lazyResetGrad implementation (#18711)
### Description
* implemented lazyResetGrad function
### Motivation and Context
* we are in the process of adding language bindings to enable training
on web
* lazyresetgrad ensures that the gradients are calculated correctly
after the first runTrainStep call
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Co-authored-by: Ashwini Khade <askhade@microsoft.com>
