Added onnxruntime check of exported ONNX, bumped up default ONNX opset
This pull request **fixes 1 alert** when merging 28bba40745713ccee63d6df39b28c15e5eb65c57 into bee43e840837d7e30e17a65b972e9fe89ecb9ca5 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-e9686dd3724e0a070a08ca4aea2cbc3518107c62)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging 73ae063885a5867b658685609129657131bf82e8 into bee43e840837d7e30e17a65b972e9fe89ecb9ca5 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-1aaf2421fba0f23d27b272e7c934c4fc9bb41ee5)

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* 1 for Unused import This pull request **fixes 1 alert** when merging b38e49ff1dfdbb7012d67b3fffcc987525d04beb into 4781d1744765fafdcd1238d8d4a9c3f94057aa23 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-d5b8fa1fdb9f0103202ad493fbbb0e7a8d01855f)

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* 1 for Unused import This pull request **fixes 1 alert** when merging de3dd297ef9f31bbb536744d4c8257c6480a9fb0 into 8ce322431a6e20c2f82e93d8a917fe2e43ad7708 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-8ccd53f48de6ae5a663d4a222ad3ab537b85da52)

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* 1 for Unused import This pull request **fixes 1 alert** when merging 5557e2e0f898f9e148c1be885a78f1abef967449 into d23ee3e3d5ed2fd17608b736ebae2b415a258a9e - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-e7af2d36f7beb394a62aa25f2579fd162fc0a7da)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging b5e4516ca2fdd8726d26934221b79b5dba4aec0d into d23ee3e3d5ed2fd17608b736ebae2b415a258a9e - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-3538ed7c1ac1590ae8f1edec3e3632c34d272536)

**fixed alerts:**

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Signed-off-by: Boris Fomitchev <bfomitchev@nvidia.com>