REF: Update optimize batch size to minimize idle cores (kernc#295)

* update batch sizes to minimize idle cores

Refs: kernc#293

Author: binarymason

backtesting/backtesting.py

@@ -1338,7 +1338,7 @@ def _optimize_grid() -> Union[pd.Series, Tuple[pd.Series, pd.Series]]:
                                     names=next(iter(param_combos)).keys()))
 
             def _batch(seq):
-                n = np.clip(len(seq) // (os.cpu_count() or 1), 5, 300)
+                n = np.clip(int(len(seq) // (os.cpu_count() or 1)), 1, 300)
                 for i in range(0, len(seq), n):
                     yield seq[i:i + n]