Adding QA Pipeline about embeddings

[ktagowski]

TODO @mkossakowski19 :

• Refactor Q&A code
• Tests on English Squad and add example of usage https://huggingface.co/datasets/squad_v2
• Tests on Polish translated Squad

[ktagowski]

TODO (17.02):

• Add tests for the QA evaluator
• Adjust tests for pipeline
  Fix errors and add more detailed metrics checks. Currently, in tests, metrics are compared as 0 < mt < 100, but it should be changed to 7 decimal precision, e.g., assert_almost_equal(mt, 0.1234567).
• Move batch encoding kwargs to LightningBasicConfig class
  Create QALightningBasicConfig that is more suited for QA (2 batch_encoding parameters and 1 QA-specific parameter (doc_stride)
• Fix linter
  There are issues with styling and pyflakes

Optional tasks (lower priority):

• [optional] Adjust LightningModule class logic
  Currently, there is code duplication between LightningModules (setup, freeze_transformer, forward). However, it needs restructuration of the inheritance hierarchy as HuggingFaceLightningModule are specific for Classification tasks.
• [optional] Notebook with QA pipeline example
  Add an example how "easy" it is to run a QA pipeline