huggingface[major]: Reduce disk footprint by 95% by making large dependencies optional (#31268)

**Description:** 
`langchain_huggingface` has a very large installation size of around 600
MB (on a Mac with Python 3.11). This is due to its dependency on
`sentence-transformers`, which in turn depends on `torch`, which is 320
MB all by itself. Similarly, the depedency on `transformers` adds
another set of heavy dependencies. With those dependencies removed, the
installation of `langchain_huggingface` only takes up ~26 MB. This is
only 5 % of the full installation!

These libraries are not necessary to use `langchain_huggingface`'s API
wrapper classes, only for local inferences/embeddings. All import
statements for those two libraries already have import guards in place
(try/catch with a helpful "please install x" message).

This PR therefore moves those two libraries to an optional dependency
group `full`. So a `pip install langchain_huggingface` will only install
the lightweight version, and a `pip install
"langchain_huggingface[full]"` will install all dependencies.

I know this may break existing code, because `sentence-transformers` and
`transformers` are now no longer installed by default. Given that users
will see helpful error messages when that happens, and the major impact
of this small change, I hope that you will still consider this PR.

**Dependencies:** No new dependencies, but new optional grouping.

Author: Simon-Stone

libs/partners/huggingface/pyproject.toml

@@ -9,8 +9,6 @@ requires-python = ">=3.9"
 dependencies = [
     "langchain-core<1.0.0,>=0.3.59",
     "tokenizers>=0.19.1",
-    "transformers>=4.39.0",
-    "sentence-transformers>=2.6.0",
     "huggingface-hub>=0.30.2",
 ]
 name = "langchain-huggingface"
@@ -23,6 +21,12 @@ readme = "README.md"
 "Release Notes" = "https://github.com/langchain-ai/langchain/releases?q=tag%3A%22langchain-huggingface%3D%3D0%22&expanded=true"
 repository = "https://github.com/langchain-ai/langchain"
 
+[project.optional-dependencies]
+full = [
+    "transformers>=4.39.0",
+    "sentence-transformers>=2.6.0",
+]
+
 [dependency-groups]
 test = [
     "pytest<8.0.0,>=7.3.0",
@@ -53,7 +57,7 @@ target-version = "py39"
 
 [tool.ruff.lint]
 select = ["E", "F", "I", "T201", "UP"]
-ignore = [ "UP007", ]
+ignore = ["UP007"]
 
 [tool.coverage.run]
 omit = ["tests/*"]