[Flamingo] Fix the memory usage of 2x checkpoint size after loading (#1201)

Co-authored-by: Martin Yuan <[email protected]>

Author: iseeyuan

torchchat/cli/builder.py

@@ -31,6 +31,8 @@
 
 from torchchat.model import Model, ModelArgs, ModelType
 
+from torchtune.modules.position_embeddings import RotaryPositionalEmbeddings
+
 from torchchat.model_config.model_config import resolve_model_config
 from torchchat.utils.build_utils import (
     device_sync,
@@ -387,9 +389,23 @@ def _load_model_default(builder_args: BuilderArgs) -> Model:
         with set_default_dtype(builder_args.precision), torch.device(
             builder_args.device
         ):
-            model = Model.from_params(builder_args.params_path)
+            # It doubles the model size the memory, with redundancies of the initialized weights.
+            # model = Model.from_params(builder_args.params_path)
+
+            # Buffers in rotary embedding are not included in the checkpoint.
+            # Instead, they are calculated in initialization. Since buffers on meta device
+            # does not host any actual values, need to reinitialize them in the actual
+            # device. Only do those buffer initialization, without initializing the entire
+            # model.
+            decoder_config = model.config.transformer_args['decoder']
+            head_dim = decoder_config['embed_dim'] // decoder_config['num_heads']
+            max_seq_len = decoder_config['max_seq_len']
+            rope_base = decoder_config['rope_base']
+            for submodule in model.modules():
+                if isinstance(submodule, RotaryPositionalEmbeddings):
+                    submodule.__init__(head_dim, max_seq_len, rope_base)
         state_dict = flamingo_meta_to_tune(checkpoint)
-        model.model.load_state_dict(state_dict)
+        model.model.load_state_dict(state_dict, assign=True, strict=False)
     else:
         checkpoint = {"model." + k: v for k, v in checkpoint.items()}
         model.load_state_dict(checkpoint, assign=True, strict=True)
@@ -472,7 +488,6 @@ def _load_model(builder_args: BuilderArgs) -> Model:
     model = model.to(device=builder_args.device, dtype=builder_args.precision)
     return model.eval()
 
-
 def _initialize_model(
     builder_args: BuilderArgs,
     quantize,