Multipack parallel bin packing (axolotl-ai-cloud#2631)

* improve readability of multipack sampler

* parallel bin packing
fix error with lambda and pickling

make sure things are in float instead of np.float

* annotations and comments update

* support for configurable group and bin size for sample packing

* fix missing map back to original indices

Author: winglian

src/axolotl/core/trainers/base.py

@@ -114,6 +114,8 @@ def _create_multipack_sampler(
             packing_efficiency_estimate=self.args.sample_packing_efficiency,
             batch_max_len=batch_max_len,
             batch_size=batch_size,
+            group_size=self.args.sample_packing_group_size,
+            bin_size=self.args.sample_packing_bin_size,
             sequential=self.args.sample_packing_sequentially,
             drop_last=True,
         )