Core: Fix __add__ for concatting two BaseMessageChunk's

[inf3rnus]

Description:

The change allows you to use the overloaded + operator correctly when +ing two BaseMessageChunk subclasses. Without this you must instantiate a subclass for it to work.

Which feels... wrong. Base classes should be decoupled from sub classes and should have in no way a dependency on them.

Issue:

You can't + a BaseMessageChunk with a BaseMessageChunk

e.g. this will explode

from langchain_core.outputs import (
    ChatGenerationChunk,
)
from langchain_core.messages import BaseMessageChunk


chunk1 = ChatGenerationChunk(
    message=BaseMessageChunk(
        type="customChunk",
        content="HI",
    ),
)

chunk2 = ChatGenerationChunk(
    message=BaseMessageChunk(
        type="customChunk",
        content="HI",
    ),
)

# this will throw
new_chunk = chunk1 + chunk2

In case anyone ran into this issue themselves, it's probably best to use the AIMessageChunk:

a la

from langchain_core.outputs import (
    ChatGenerationChunk,
)
from langchain_core.messages import AIMessageChunk


chunk1 = ChatGenerationChunk(
    message=AIMessageChunk(
        content="HI",
    ),
)

chunk2 = ChatGenerationChunk(
    message=AIMessageChunk(
        content="HI",
    ),
)

# No explosion!
new_chunk = chunk1 + chunk2

Dependencies:

None!

Twitter handle:
aaron_vogler

Keeping these for later if need be:

baskaryan
efriis 
eyurtsev
ccurme 
vbarda
hwchase17
baskaryan
efriis

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