### Optimize computation orders
In `Roberta/Electra`, when `ClassificationHead` is used, there is
slicing operation on features on sequence_length dimensions, then loss
calculations only depend on this sliced data. This is a slicing at axis
1. Before slicing the shape is [batch, sequence_length, hidden], after
slicing, it becomes [batch , hidden_stage]
We had opportunities to bring this slicing earlier as much as possible,
by passing through simple elementwise ops (like Add/Div), or
Layernorm/Softmax(if their reduce axis is after the slicing axis), or
even MatMul's the left operand (if only it did not affect the last
dims).
For operators like Reshape/Transpose, it is special since they have
either data specified (after slicing we need update), or they have perm
specified, which requires the input rank remain unchanged. So for those
kinds of operators, we can remain the original rank, but just leave the
sliced dim to be 1, after the compute completed, we do a Squeeze.
```
class RobertaClassificationHead(nn.Module):
"""Head for sentence-level classification tasks."""
def __init__(self, config):
super().__init__()
self.dense = nn.Linear(config.hidden_size, config.hidden_size)
classifier_dropout = (
config.classifier_dropout if config.classifier_dropout is not None else config.hidden_dropout_prob
)
self.dropout = nn.Dropout(classifier_dropout)
self.out_proj = nn.Linear(config.hidden_size, config.num_labels)
def forward(self, features, **kwargs):
x = features[:, 0, :] # take <s> token (equiv. to [CLS])
x = self.dropout(x)
x = self.dense(x)
x = torch.tanh(x)
x = self.dropout(x)
x = self.out_proj(x)
return x
```
src\transformers\models\roberta\modeling_roberta.py
src\transformers\models\electra\modeling_electra.py
#### Benchmark
A simple benchmark shows Robeta training latency dropped from 208ms ~
199ms. 4.5+% reduction.
More comprehensive tests are on the way.
### Motivation and Context
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### Add guidelines for ORTModule
As title.
Feel free to let me know if I missed something.
### Motivation and Context
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