Make it possible to specify multiple config files

Summary:

Test Plan:

Test that this iterpolates in the right order, config -> configs -> cli args

```
# All three sources
python -m bytelatent.print_config config=bytelatent/configs/debug.yaml configs=[internal/configs/s3_debug.yaml] eval=null

# What worked before
python -m bytelatent.print_config config=internal/configs/s3_debug.yaml eval=null
```

bytelatent/args.py

@@ -40,12 +40,23 @@ def get_rng_state(seed: int, rank: int, world_size: int) -> dict[str, Any]:
 
 def parse_args(args_cls):
     cli_args = OmegaConf.from_cli()
-    file_cfg = OmegaConf.load(cli_args.config)
-    # We remove 'config' attribute from config as the underlying DataClass does not have it
-    del cli_args.config
+    file_cfgs = []
+    if "config" in cli_args:
+        file_cfg = OmegaConf.load(cli_args["config"])
+        del cli_args["config"]
+        file_cfgs.append(file_cfg)
+
+    if "configs" in cli_args:
+        for c in cli_args["configs"]:
+            extra_file_cfg = OmegaConf.load(c)
+            file_cfgs.append(extra_file_cfg)
+        del cli_args["configs"]
 
     default_cfg = OmegaConf.create(args_cls().model_dump())
-    cfg = OmegaConf.merge(default_cfg, file_cfg, cli_args)
+    to_merge = [default_cfg]
+    to_merge.extend(file_cfgs)
+    to_merge.append(cli_args)
+    cfg = OmegaConf.merge(*to_merge)
     cfg = OmegaConf.to_container(cfg, resolve=True, throw_on_missing=True)
     pydantic_args = args_cls.model_validate(cfg)
     return pydantic_args

bytelatent/configs/debug.yaml

@@ -56,13 +56,11 @@ model:
   recompute_attn: false
   custom_bwd: false
   layer_ckpt: "none"
-  patch_only_encoder: false
-  patch_only_decoder: false
   use_local_encoder_transformer: true
   init_use_gaussian: true
   init_use_depth: "current"
-  attn_bias_type: "block_causal"
   attn_impl: "xformers"
+  attn_bias_type: "block_causal"
   alpha_depth: "disabled"
   max_length: 256
   local_attention_window_len: 512

bytelatent/print_config.py

@@ -0,0 +1,10 @@
+from bytelatent.args import TrainArgs, parse_args
+
+
+def main():
+    train_args = parse_args(TrainArgs)
+    print(train_args.model_dump_json(indent=4))
+
+
+if __name__ == "__main__":
+    main()