Megatron-lm model parallel gradient clipping
This pull request **introduces 3 alerts** when merging 9cc1d68444b748be950f0ae0b4e4c7701320bb71 into 297a8bdbb0370a51cb74a41288025befd9cd2392 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-8b83c948558d1688357dbe7f12b05949d53d236d)

**new alerts:**

* 2 for Unused import
* 1 for Except block handles &#39;BaseException&#39; This pull request **introduces 3 alerts** when merging 531b79ef3d1f704acb529e223241b2fbbe1b1827 into f58ab531376a854e2e94b0fedc05705bc3711086 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-17dcebf23511e395c6b1753a47d43d31b05f9635)

**new alerts:**

* 2 for Unused import
* 1 for Except block handles &#39;BaseException&#39; This pull request **introduces 3 alerts** when merging 5ccfc94ad6fde36dfabebae5d355b0687ef2adbf into 212b1f343aea96bb390a1e00bdcea5b1a9afe1fc - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-9d071914f4135f4c7fbf73b1dde5dd7054694de2)

**new alerts:**

* 2 for Unused import
* 1 for Except block handles &#39;BaseException&#39; This pull request **introduces 3 alerts** when merging cc95c613dcb1d2e04e607a82ba898f5c49956db0 into d8150b41bbba0c2ee2e291b309651cef2ed248bf - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-71feca283519465f203dd8bb933e86050cabce60)

**new alerts:**

* 2 for Unused import
* 1 for Except block handles &#39;BaseException&#39;
This PR adds model parallel gradient clipping to `NLPModel`.