Fix GPU usage by Khoj on Macs to speed up search and indexing

- Ensure all tensors are on MPS device before doing operations across them

- Background
  - GPU is used by default for Khoj on MacOS now
    - Needed PyTorch > 1.13.0 on Macs to use GPU, which we do now
  - MPS should speed up search and indexing on MacOS

src/search_type/text_search.py

@@ -68,7 +68,7 @@ def compute_embeddings(entries_with_ids: list[tuple[int, Entry]], bi_encoder, em
         if new_entries:
             new_embeddings = bi_encoder.encode(new_entries, convert_to_tensor=True, device=state.device, show_progress_bar=True)
             existing_entry_ids = [id for id, _ in entries_with_ids if id != -1]
-            existing_embeddings = torch.index_select(corpus_embeddings, 0, torch.tensor(existing_entry_ids)) if existing_entry_ids else torch.Tensor()
+            existing_embeddings = torch.index_select(corpus_embeddings, 0, torch.tensor(existing_entry_ids, device=state.device)) if existing_entry_ids else torch.Tensor(device=state.device)
             corpus_embeddings = torch.cat([existing_embeddings, new_embeddings], dim=0)
     # Else compute the corpus embeddings from scratch
     else:
@@ -102,12 +102,12 @@ def query(raw_query: str, model: TextSearchModel, rank_results=False):
     else:
         start = time.time()
         entries = [entries[id] for id in included_entry_indices]
-        corpus_embeddings = torch.index_select(corpus_embeddings, 0, torch.tensor(list(included_entry_indices)))
+        corpus_embeddings = torch.index_select(corpus_embeddings, 0, torch.tensor(list(included_entry_indices), device=state.device))
         end = time.time()
         logger.debug(f"Keep entries satisfying all filters: {end - start} seconds")
 
     end_filter = time.time()
-    logger.debug(f"Total Filter Time: {end_filter - start_filter:.3f} seconds")
+    logger.debug(f"Total Filter Time: {end_filter - start_filter:.3f} seconds on device: {state.device}")
 
     if entries is None or len(entries) == 0:
         return [], []