feat(ollama): add dimensions to OllamaEmbeddings (#36543)

Fixes #34623

Add `dimensions` field to `OllamaEmbeddings` to allow users to specify 
output embedding size for models that support variable dimensions . The
field is passed
directly to the Ollama client's `embed()` call for both sync and async
methods.

**How I verified it works:**
- Ran unit tests: `python -m pytest tests/unit_tests/ -v`
- Ran integration tests against a live Ollama instance:
`OLLAMA_HOST=http://ollama:11434 python -m pytest
tests/integration_tests/ -v`
- Confirmed that passing `dimensions=768` no longer raises
`extra_forbidden`
  Pydantic validation error and returns embeddings of the expected size.

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>
Co-authored-by: Mason Daugherty <github@mdrxy.com>

libs/partners/ollama/langchain_ollama/embeddings.py

@@ -6,7 +6,13 @@ from typing import Any
 
 from langchain_core.embeddings import Embeddings
 from ollama import AsyncClient, Client
-from pydantic import BaseModel, ConfigDict, PrivateAttr, model_validator
+from pydantic import (
+    BaseModel,
+    ConfigDict,
+    PrivateAttr,
+    field_validator,
+    model_validator,
+)
 from typing_extensions import Self
 
 from langchain_ollama._utils import (
@@ -124,6 +130,20 @@ class OllamaEmbeddings(BaseModel, Embeddings):
     model: str
     """Model name to use."""
 
+    dimensions: int | None = None
+    """Number of dimensions for the output embedding vectors.
+
+    If not provided, the model's default embedding dimensionality is used.
+    """
+
+    @field_validator("dimensions")
+    @classmethod
+    def _validate_dimensions(cls, v: int | None) -> int | None:
+        if v is not None and v < 1:
+            msg = "`dimensions` must be a positive integer."
+            raise ValueError(msg)
+        return v
+
     validate_model_on_init: bool = False
     """Whether to validate the model exists in ollama locally on initialization.
 
@@ -303,7 +323,11 @@ class OllamaEmbeddings(BaseModel, Embeddings):
             )
             raise RuntimeError(msg)
         return self._client.embed(
-            self.model, texts, options=self._default_params, keep_alive=self.keep_alive
+            self.model,
+            texts,
+            dimensions=self.dimensions,
+            options=self._default_params,
+            keep_alive=self.keep_alive,
         )["embeddings"]
 
     def embed_query(self, text: str) -> list[float]:
@@ -322,6 +346,7 @@ class OllamaEmbeddings(BaseModel, Embeddings):
             await self._async_client.embed(
                 self.model,
                 texts,
+                dimensions=self.dimensions,
                 options=self._default_params,
                 keep_alive=self.keep_alive,
             )