core: add additional import mappings to loads (#26406)

Support using additional import mapping. This allows users to override
old mappings/add new imports to the loads function.

- [x ] **Add tests and docs**: If you're adding a new integration,
please include
1. a test for the integration, preferably unit tests that do not rely on
network access,
2. an example notebook showing its use. It lives in
`docs/docs/integrations` directory.


- [ x] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

libs/core/tests/unit_tests/load/test_serializable.py

@@ -1,6 +1,6 @@
 from typing import Dict
 
-from langchain_core.load import Serializable, dumpd
+from langchain_core.load import Serializable, dumpd, load
 from langchain_core.load.serializable import _is_field_useful
 from langchain_core.pydantic_v1 import Field
 
@@ -107,3 +107,61 @@ def test__is_field_useful() -> None:
     foo = Foo(x=default_x, y=default_y, z=ArrayObj())
     assert not _is_field_useful(foo, "x", foo.x)
     assert not _is_field_useful(foo, "y", foo.y)
+
+
+class Foo(Serializable):
+    bar: int
+    baz: str
+
+    @classmethod
+    def is_lc_serializable(cls) -> bool:
+        return True
+
+
+def test_simple_deserialization() -> None:
+    foo = Foo(bar=1, baz="hello")
+    assert foo.lc_id() == ["tests", "unit_tests", "load", "test_serializable", "Foo"]
+    serialized_foo = dumpd(foo)
+    assert serialized_foo == {
+        "id": ["tests", "unit_tests", "load", "test_serializable", "Foo"],
+        "kwargs": {"bar": 1, "baz": "hello"},
+        "lc": 1,
+        "type": "constructor",
+    }
+    new_foo = load(serialized_foo, valid_namespaces=["tests"])
+    assert new_foo == foo
+
+
+class Foo2(Serializable):
+    bar: int
+    baz: str
+
+    @classmethod
+    def is_lc_serializable(cls) -> bool:
+        return True
+
+
+def test_simple_deserialization_with_additional_imports() -> None:
+    foo = Foo(bar=1, baz="hello")
+    assert foo.lc_id() == ["tests", "unit_tests", "load", "test_serializable", "Foo"]
+    serialized_foo = dumpd(foo)
+    assert serialized_foo == {
+        "id": ["tests", "unit_tests", "load", "test_serializable", "Foo"],
+        "kwargs": {"bar": 1, "baz": "hello"},
+        "lc": 1,
+        "type": "constructor",
+    }
+    new_foo = load(
+        serialized_foo,
+        valid_namespaces=["tests"],
+        additional_import_mappings={
+            ("tests", "unit_tests", "load", "test_serializable", "Foo"): (
+                "tests",
+                "unit_tests",
+                "load",
+                "test_serializable",
+                "Foo2",
+            )
+        },
+    )
+    assert isinstance(new_foo, Foo2)