Use ONNX / Core ML compatible method to broadcast (open-mmlab#310)

* Use ONNX / Core ML compatible method to broadcast.

Unfortunately `tile` could not be used either, it's still not compatible
with ONNX.

See open-mmlab#284.

* Add comment about why broadcast_to is not used.

Also, apply style to changed files.

* Make sure broadcast remains in same device.

src/diffusers/models/unet_2d.py

@@ -120,7 +120,6 @@ def __init__(
     def forward(
         self, sample: torch.FloatTensor, timestep: Union[torch.Tensor, float, int]
     ) -> Dict[str, torch.FloatTensor]:
-
         # 0. center input if necessary
         if self.config.center_input_sample:
             sample = 2 * sample - 1.0
@@ -132,8 +131,8 @@ def forward(
         elif torch.is_tensor(timesteps) and len(timesteps.shape) == 0:
             timesteps = timesteps[None].to(sample.device)
 
-        # broadcast to batch dimension
-        timesteps = timesteps.broadcast_to(sample.shape[0])
+        # broadcast to batch dimension in a way that's compatible with ONNX/Core ML
+        timesteps = timesteps * torch.ones(sample.shape[0], dtype=timesteps.dtype, device=timesteps.device)
 
         t_emb = self.time_proj(timesteps)
         emb = self.time_embedding(t_emb)

src/diffusers/models/unet_2d_condition.py

@@ -121,7 +121,6 @@ def forward(
         timestep: Union[torch.Tensor, float, int],
         encoder_hidden_states: torch.Tensor,
     ) -> Dict[str, torch.FloatTensor]:
-
         # 0. center input if necessary
         if self.config.center_input_sample:
             sample = 2 * sample - 1.0
@@ -133,8 +132,8 @@ def forward(
         elif torch.is_tensor(timesteps) and len(timesteps.shape) == 0:
             timesteps = timesteps[None].to(sample.device)
 
-        # broadcast to batch dimension
-        timesteps = timesteps.broadcast_to(sample.shape[0])
+        # broadcast to batch dimension in a way that's compatible with ONNX/Core ML
+        timesteps = timesteps * torch.ones(sample.shape[0], dtype=timesteps.dtype, device=timesteps.device)
 
         t_emb = self.time_proj(timesteps)
         emb = self.time_embedding(t_emb)
@@ -145,7 +144,6 @@ def forward(
         # 3. down
         down_block_res_samples = (sample,)
         for downsample_block in self.down_blocks:
-
             if hasattr(downsample_block, "attentions") and downsample_block.attentions is not None:
                 sample, res_samples = downsample_block(
                     hidden_states=sample, temb=emb, encoder_hidden_states=encoder_hidden_states
@@ -160,7 +158,6 @@ def forward(
 
         # 5. up
         for upsample_block in self.up_blocks:
-
             res_samples = down_block_res_samples[-len(upsample_block.resnets) :]
             down_block_res_samples = down_block_res_samples[: -len(upsample_block.resnets)]