Use cross-encoder to rerank search results by default on GPU machines

Latest sentence-transformer package uses GPU for cross-encoder. This
makes it fast enough to enable reranking on machines with GPU.

Enabling search reranking by default allows (at least) users with GPUs
to side-step learning the UI affordance to rerank results
(i.e hitting Cmd/Ctrl-Enter or ENTER).

pyproject.toml

@@ -50,7 +50,7 @@ dependencies = [
     "pyyaml == 6.0",
     "rich >= 13.3.1",
     "schedule == 1.1.0",
-    "sentence-transformers == 2.3.1",
+    "sentence-transformers == 2.5.1",
     "transformers >= 4.28.0",
     "torch == 2.0.1",
     "uvicorn == 0.17.6",

src/khoj/search_type/text_search.py

@@ -177,8 +177,9 @@ def deduplicated_search_responses(hits: List[SearchResponse]):
 
 
 def rerank_and_sort_results(hits, query, rank_results, search_model_name):
-    # If we have more than one result and reranking is enabled
-    rank_results = rank_results and len(list(hits)) > 1
+    # Rerank results if explicitly requested or if device has GPU
+    # AND if we have more than one result
+    rank_results = (rank_results or state.device.type != "cpu") and len(list(hits)) > 1
 
     # Score all retrieved entries using the cross-encoder
     if rank_results:

src/khoj/utils/helpers.py

@@ -331,7 +331,12 @@ def batcher(iterable, max_n):
         yield (x for x in chunk if x is not None)
 
 
+def is_env_var_true(env_var: str, default: str = "false") -> bool:
+    """Get state of boolean environment variable"""
+    return os.getenv(env_var, default).lower() == "true"
+
+
 def in_debug_mode():
     """Check if Khoj is running in debug mode.
     Set KHOJ_DEBUG environment variable to true to enable debug mode."""
-    return os.getenv("KHOJ_DEBUG", "false").lower() == "true"
+    return is_env_var_true("KHOJ_DEBUG")