create configuration file for MaMMUT training (#521)

Summary:
Pull Request resolved: #521

Mostly based on original coca and https://github.com/lucidrains/MaMMUT-pytorch

Update the logics of loading checkpoint for MaMMUT text decoder as well.

Differential Revision:
D52891614

Privacy Context Container: 303860477774201

fbshipit-source-id: 192a1826fd59a80bf99e8545408e19938069a599

torchmultimodal/modules/layers/transformer.py

@@ -412,10 +412,7 @@ def _forward_prenorm(
         self_attn_output = attn_output + hidden_states
 
         # Optional cross-attention
-        if self.use_cross_attention:
-            assert (
-                encoder_hidden_states is not None
-            ), "encoder_hidden_states must be provided for cross attention"
+        if self.use_cross_attention and encoder_hidden_states is not None:
             assert hasattr(
                 self, "cross_attention_layernorm"
             ), "Cross-attention layernorm not initialized"