llamacpp wrong default value passed for `f16_kv` (#3320)

Fixes default f16_kv value in llamacpp; corrects incorrect parameter
passed.

See:
ba3959eafd/llama_cpp/llama.py (L33)

Fixes #3241
Fixes #3301

langchain/llms/llamacpp.py

@@ -37,7 +37,7 @@ class LlamaCpp(LLM):
     seed: int = Field(-1, alias="seed")
     """Seed. If -1, a random seed is used."""
 
-    f16_kv: bool = Field(False, alias="f16_kv")
+    f16_kv: bool = Field(True, alias="f16_kv")
     """Use half-precision for key/value cache."""
 
     logits_all: bool = Field(False, alias="logits_all")