Moving calculating the class weights to the data loader creation method.
This pull request **fixes 3 alerts** when merging 0bc9d88cc28dbdb604348502888931556acde702 into 7d8b7d679a2a4c9d3a6b2d8def8888e53c804ded - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-1f5d9f48dd6cb74ec4d5390dc1d8290bf990ed9d)

**fixed alerts:**

* 2 for Unused local variable
* 1 for Unused import This pull request **fixes 2 alerts** when merging 4715c44a944ff344f6fb967393df078aea321216 into c5cd85f4202a94a44aba3103628fc39d73c48379 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-c0f6315b050920f0072cc5321f3a29a7b3a0b65d)

**fixed alerts:**

* 2 for Unused local variable Can we still make changes to b4 branch? @tkornuta-nvidia @okuchaiev ? > 
> 
> Can we still make changes to b4 branch? @tkornuta-nvidia @okuchaiev ?

Which branch should we use for bug fixes needed for TLT? This pull request **fixes 2 alerts** when merging af60927630e6ae145748eb9ef73ba7e82172a714 into c5cd85f4202a94a44aba3103628fc39d73c48379 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-ba6fbbbe145ae3615601b252ce75d1d24357b625)

**fixed alerts:**

* 2 for Unused local variable
It is necessary to support class_balancing in TTL.