Do not materialize mlp_head if no field_heads (#2457)

In case it is used in a proposal network we avoid initializing the head.

Co-authored-by: Matthew Tancik <matt@lumalabs.ai>

nerfstudio/fields/vanilla_nerf_field.py

@@ -74,15 +74,15 @@ class NeRFField(Field):
             skip_connections=skip_connections,
             out_activation=nn.ReLU(),
         )
-
-        self.mlp_head = MLP(
-            in_dim=self.mlp_base.get_out_dim() + self.direction_encoding.get_out_dim(),
-            num_layers=head_mlp_num_layers,
-            layer_width=head_mlp_layer_width,
-            out_activation=nn.ReLU(),
-        )
-
         self.field_output_density = DensityFieldHead(in_dim=self.mlp_base.get_out_dim())
+
+        if field_heads:
+            self.mlp_head = MLP(
+                in_dim=self.mlp_base.get_out_dim() + self.direction_encoding.get_out_dim(),
+                num_layers=head_mlp_num_layers,
+                layer_width=head_mlp_layer_width,
+                out_activation=nn.ReLU(),
+            )
         self.field_heads = nn.ModuleList([field_head() for field_head in field_heads] if field_heads else [])  # type: ignore
         for field_head in self.field_heads:
             field_head.set_in_dim(self.mlp_head.get_out_dim())  # type: ignore