Fixes perplexity computation (issue https://github.com/NVIDIA/NeMo/issues/1289)
This pull request **fixes 1 alert** when merging eb11ff4b16f70648862ee2e85a4912101d10c1d6 into 1ac828df423fbcec1b34c650b3a20266bb133dde - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-f332123bfafc9c57f8b57f0fc73bcbf047aea797)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging 89bc627ed6df26dd20471ee7a848de79964d6a84 into 1ac828df423fbcec1b34c650b3a20266bb133dde - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-aca3524081cb9dd1c26ea5a0f50b3bfd93848444)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging fbc26fc722bda9ff51a5c5ce44b8b59dc5c0e4f6 into 1ac828df423fbcec1b34c650b3a20266bb133dde - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-88d367f4a639ea975e3a703a26cc799f11241726)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging f5b54e58748917389b1792c99bea5be131236709 into 1ac828df423fbcec1b34c650b3a20266bb133dde - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-88c4f53ef90666cccb4e8d4dfc2d679324a9cb73)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging 0703eba7f634fd3423b0b0d30272028077a63ec0 into 1ac828df423fbcec1b34c650b3a20266bb133dde - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-24e865175ba9a1ebfd8b03c56148a767751006d4)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging d0c1a2e10d8524583a92aba4b6fd8e5e3b631ffe into 1ac828df423fbcec1b34c650b3a20266bb133dde - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-c97c4bbc694eeee169f495a6cd9222885c188651)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging 40e0096fcf99f77dea642bf885dbda79956927bf into 1ac828df423fbcec1b34c650b3a20266bb133dde - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-8955cc980ad0768de00b3dcfab9b90294ca9bc8a)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging bfd10487c05c6d22c49c842e5791d2806b098d23 into d28114ae203558a0bd1bcc69e78aeb957ea9ed5e - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-bf54d165f39cdd3449a151f459152c3e482f24bc)

**fixed alerts:**

* 1 for Unused import @PeganovAnton Could you add the Perplexity metric to the Transformer based language model class in NeMo ?  This pull request **fixes 1 alert** when merging f2441874c6b42c078e42e3ab16ca3e04a0fb7255 into 38c854da3f553b6577d6f8b08490e026b53b7ea3 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-2a522744379d24a0f18b1737749d2b04e7ad0e42)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging 147aa9d37029b43cfb3be7a9d0c43655448a0219 into 38c854da3f553b6577d6f8b08490e026b53b7ea3 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-310184ee2f5eda2471ca8fdeb3289a3a75529a5b)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging 830b46987f2d3d40462f44c78f9c8ccb5b4cddf0 into 38c854da3f553b6577d6f8b08490e026b53b7ea3 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-ec9a01906d2dd5159d6321ad9bfeab82823d5c8e)

**fixed alerts:**

* 1 for Unused import This pull request **fixes 1 alert** when merging 07414ccd56c07724b1c16b59800cb72ebbaca0b7 into 075472199a3b2b65530e232c6ece4ce6ef92bf28 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-2199a3ee791caa50fd1a5dad77f2dd2d5f57904d)

**fixed alerts:**

* 1 for Unused import
* Add perplexity draft

Signed-off-by: PeganovAnton <peganoff2@mail.ru>

* Add perplexity metric

Signed-off-by: PeganovAnton <peganoff2@mail.ru>

* Add docstrings and copyright, fix comments

Signed-off-by: PeganovAnton <peganoff2@mail.ru>

* Fix code style

Signed-off-by: PeganovAnton <peganoff2@mail.ru>

* Fix code style

Signed-off-by: PeganovAnton <peganoff2@mail.ru>

* Improve docstring

Signed-off-by: PeganovAnton <peganoff2@mail.ru>

* Fix docstring hrefs

Signed-off-by: PeganovAnton <peganoff2@mail.ru>