[RAG/FiD] Support Left Padded Inputs

[klshuster]

Patch description
This patch provides support for RAG and FiD with left-padded input, if desired.

Two methods are required to change:

1. RagModel.concat_docs_and_input - when left padding, we need to make sure that the concatenated docs and input are padded appropriately
2. concat_enc_outs - When concatenating encoder outputs for FiD, we need to make sure that we are concatenating the correct parts of the input.

Testing steps
Added CI test.

$ pytest -k TestLeftPadding
=====test session starts =====
platform linux -- Python 3.7.9, pytest-5.3.2, py-1.10.0, pluggy-0.13.1
rootdir: /private/home/kshuster/ParlAI, inifile: pytest.ini, testpaths: tests, parlai/tasks
plugins: hydra-core-1.1.0, requests-mock-1.7.0, regressions-2.1.1, datadir-1.3.1
collected 1312 items / 1310 deselected / 2 selected

tests/nightly/gpu/test_rag.py ..                                                                                                                                              [100%]

====slowest 10 test durations ====
10.29s call     tests/nightly/gpu/test_rag.py::TestLeftPadding::test_concat_docs_and_input

(0.00 durations hidden.  Use -vv to show these durations.)
====2 passed, 1310 deselected, 4 warnings in 21.34s ====

[stephenroller]

thx for tests