Add create_conv_retrieval_chain func
Labels: name: auto:improvement, name: area: models, name: size:L

```
                                                     +----------+
                                                     | MapInput |
                                                   **+----------+****
                                               ****                  ****
                                           ****                          ***
                                         **                                 ****
                  +------------------------------------+                        **
                  | Lambda(itemgetter('chat_history')) |                         *
                  +------------------------------------+                         *
                                     *                                           *
                                     *                                           *
                                     *                                           *
                       +---------------------------+            +--------------------------------+
                       | Lambda(_get_chat_history) |            | Lambda(itemgetter('question')) |
                       +---------------------------+            +--------------------------------+
                                     *                                           *
                                     *                                           *
                                     *                                           *
                      +----------------------------+                +------------------------+
                      | ContextSet('chat_history') |                | ContextSet('question') |
                      +----------------------------+                +------------------------+
                                               ****                  ****
                                                   ****          ****
                                                       **      **
                                                     +-----------+
                                                     | MapOutput |
                                                     +-----------+
                                                           *
                                                           *
                                                           *
                                                  +----------------+
                                                  | PromptTemplate |
                                                  +----------------+
                                                           *
                                                           *
                                                           *
                                                    +-------------+
                                                    | FakeListLLM |
                                                    +-------------+
                                                           *
                                                           *
                                                           *
                                                  +-----------------+
                                                  | StrOutputParser |
                                                  +-----------------+
                                                           *
                                                           *
                                                           *
                                            +----------------------------+
                                            | ContextSet('new_question') |
                                            +----------------------------+
                                                           *
                                                           *
                                                           *
                                                +---------------------+
                                                | SequentialRetriever |
                                                +---------------------+
                                                           *
                                                           *
                                                           *
                                        +------------------------------------+
                                        | Lambda(_reduce_tokens_below_limit) |
                                        +------------------------------------+
                                                           *
                                                           *
                                                           *
                                           +-------------------------------+
                                           | ContextSet('input_documents') |
                                           +-------------------------------+
                                                           *
                                                           *
                                                           *
                                                     +----------+
                                                  ***| MapInput |****
                                           *******   +----------+    ********
                                   ********                *                 *******
                            *******                         *                       ********
                        ****                                *                               ****
+-------------------------------+            +----------------------------+            +----------------------------+
| ContextGet('input_documents') |            | ContextGet('chat_history') |            | ContextGet('new_question') |
+-------------------------------+****        +----------------------------+            +----------------------------+
                                     *********                *                 *******
                                              ********         *          ******
                                                      *****    *      ****
                                                         +-----------+
                                                         | MapOutput |
                                                         +-----------+
                                                                *
                                                                *
                                                                *
                                                        +-------------+
                                                        | FakeListLLM |
                                                        +-------------+
                                                                *
                                                                *
                                                                *
                                                          +----------+
                                                       ***| MapInput |***
                                               ********   +----------+   ******
                                        *******                 *              *****
                                ********                        *                   ******
                            ****                                *                         ***
    +-------------------------------+            +----------------------------+            +-------------+
    | ContextGet('input_documents') |            | ContextGet('new_question') |          **| Passthrough |
    +-------------------------------+            +----------------------------+   *******  +-------------+
                                     *******                 *              ******
                                            ******           *       *******
                                                  ****      *    ****
                                                     +-----------+
                                                     | MapOutput |
                                                     +-----------+
```