fix(core): lazy loading transformers, numpy, simsimd

[LennyMalcolm0]

fix(core): lazy loading transformers, numpy, simsimd

Summary

Optimized the initial import speed of langchain_core by converting top-level imports of transformers, numpy, and simsimd into lazy, local imports.

Details

Confirmed with python -X importtime that these dependencies are no longer loaded on initial import, and verified functionality through make format/lint and est.

Disclaimer

I used the Gemini CLI to help with the specific implementation details and to ensure compliance with linting and type checking and drafting this PR description.

before
without transformers / torch env.

$ time uv run python -c "from langchain_core.prompts import BasePromptTemplate"
real    0m0.600s
user    0m0.509s
sys     0m0.074s

with transformers / torch installed env.

$ uv pip install transformers
...
$ uv pip install numpy
...

$ time uv run python -c "from langchain_core.prompts import BasePromptTemplate"
real    0m9.664s
user    0m4.268s
sys     0m0.752s

$ time uv run python -c "from langchain_core.prompts import BasePromptTemplate" # one more time

real    0m4.030s
user    0m3.877s
sys     0m0.468s

after PR applied

$ time uv run python -c "from langchain_core.prompts import BasePromptTemplate"
real    0m0.797s
user    0m0.662s
sys     0m0.110s

$ uv pip show transformers
Name: transformers
Version: 4.55.0
Location: /home/future/repo/linkBrain-server/.venv/lib/python3.13/site-packages
Requires: filelock, huggingface-hub, numpy, packaging, pyyaml, regex, requests, safetensors, tokenizers, tqdm
Required-by: docling-ibm-models, optimum, sentence-transformers

See also langchain-ai#35469

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Cloned from langchain-ai#35466 for DevAsign stress testing.