Addition of a DialoguePipeline (#5516)

* initial commit for pipeline implementation

Addition of input processing and history concatenation

* Conversation pipeline tested and working for single & multiple conversation inputs

* Added docstrings for dialogue pipeline

* Addition of dialogue pipeline integration tests

* Delete test_t5.py

* Fixed max code length

* Updated styling

* Fixed test broken by formatting tools

* Removed unused import

* Added unit test for DialoguePipeline

* Fixed Tensorflow compatibility

* Fixed multi-framework support using framework flag

* - Fixed docstring
- Added `min_length_for_response` as an initialization parameter
- Renamed `*args` to `conversations`, `conversations` being a `Conversation` or a `List[Conversation]`
- Updated truncation to truncate entire segments of conversations, instead of cutting in the middle of a user/bot input

* - renamed pipeline name from dialogue to conversational
- removed hardcoded default value of 1000 and use config.max_length instead
- added `append_response` and `set_history` method to the Conversation class to avoid direct fields mutation
- fixed bug in history truncation method

* - Updated ConversationalPipeline to accept only active conversations (otherwise a ValueError is raised)

* - Simplified input tensor conversion

* - Updated attention_mask value for Tensorflow compatibility

* - Updated last dialogue reference to conversational & fixed integration tests

* Fixed conflict with master

* Updates following review comments

* Updated formatting

* Added Conversation and ConversationalPipeline to the library __init__, addition of docstrings for Conversation, added both to the docs

* Update src/transformers/pipelines.py

Updated docsting following review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

docs/source/main_classes/pipelines.rst

@@ -71,3 +71,11 @@ TextGenerationPipeline
 ==========================================
 
 .. autoclass:: transformers.TextGenerationPipeline
+
+
+ConversationalPipeline
+==========================================
+
+.. autoclass:: transformers.Conversation
+
+.. autoclass:: transformers.ConversationalPipeline