Implementation of activations as pytorch modules (#15616)

* Implement activations as pytorch modules

* Apply fixup

* Add missing tests for activations

* Update docstring
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

src/transformers/activations.py

@@ -16,7 +16,7 @@ import math
 
 import torch
 from packaging import version
-from torch import nn
+from torch import Tensor, nn
 
 from .utils import logging
 
@@ -24,39 +24,66 @@ from .utils import logging
 logger = logging.get_logger(__name__)
 
 
-def gelu_python(x):
+class NewGELUActivation(nn.Module):
+    """
+    Implementation of the GELU activation function currently in Google BERT repo (identical to OpenAI GPT). Also see
+    the Gaussian Error Linear Units paper: https://arxiv.org/abs/1606.08415
+    """
+
+    def __init__(self):
+        super().__init__()
+
+    def forward(self, input: Tensor) -> Tensor:
+        return 0.5 * input * (1.0 + torch.tanh(math.sqrt(2.0 / math.pi) * (input + 0.044715 * torch.pow(input, 3.0))))
+
+
+class GELUActivation(nn.Module):
     """
     Original Implementation of the GELU activation function in Google BERT repo when initially created. For
     information: OpenAI GPT's GELU is slightly different (and gives slightly different results): 0.5 * x * (1 +
     torch.tanh(math.sqrt(2 / math.pi) * (x + 0.044715 * torch.pow(x, 3)))) This is now written in C in nn.functional
     Also see the Gaussian Error Linear Units paper: https://arxiv.org/abs/1606.08415
     """
-    return x * 0.5 * (1.0 + torch.erf(x / math.sqrt(2.0)))
 
+    def __init__(self, use_gelu_python: bool = False):
+        super().__init__()
+        if version.parse(torch.__version__) < version.parse("1.4") or use_gelu_python:
+            self.act = self._gelu_python
+        else:
+            self.act = nn.functional.gelu
+
+    def _gelu_python(self, input: Tensor) -> Tensor:
+        return input * 0.5 * (1.0 + torch.erf(input / math.sqrt(2.0)))
 
-def gelu_new(x):
+    def forward(self, input: Tensor) -> Tensor:
+        return self.act(input)
+
+
+class FastGELUActivation(nn.Module):
     """
-    Implementation of the GELU activation function currently in Google BERT repo (identical to OpenAI GPT). Also see
-    the Gaussian Error Linear Units paper: https://arxiv.org/abs/1606.08415
+    Applies GELU approximation that is slower than QuickGELU but more accurate. See: https://github.com/hendrycks/GELUs
     """
-    return 0.5 * x * (1.0 + torch.tanh(math.sqrt(2.0 / math.pi) * (x + 0.044715 * torch.pow(x, 3.0))))
 
+    def __init__(self):
+        super().__init__()
 
-if version.parse(torch.__version__) < version.parse("1.4"):
-    gelu = gelu_python
-else:
-    gelu = nn.functional.gelu
+    def forward(self, input: Tensor) -> Tensor:
+        return 0.5 * input * (1.0 + torch.tanh(input * 0.7978845608 * (1.0 + 0.044715 * input * input)))
 
 
-def gelu_fast(x):
-    return 0.5 * x * (1.0 + torch.tanh(x * 0.7978845608 * (1.0 + 0.044715 * x * x)))
+class QuickGELUActivation(nn.Module):
+    """
+    Applies GELU approximation that is fast but somewhat inaccurate. See: https://github.com/hendrycks/GELUs
+    """
 
+    def __init__(self):
+        super().__init__()
 
-def quick_gelu(x):
-    return x * torch.sigmoid(1.702 * x)
+    def forward(self, input: Tensor) -> Tensor:
+        return input * torch.sigmoid(1.702 * input)
 
 
-def _silu_python(x):
+class SiLUActivation(nn.Module):
     """
     See Gaussian Error Linear Units (Hendrycks et al., https://arxiv.org/abs/1606.08415) where the SiLU (Sigmoid Linear
     Unit) was originally introduced and coined, and see Sigmoid-Weighted Linear Units for Neural Network Function
@@ -64,46 +91,65 @@ def _silu_python(x):
     Activation Function (Ramachandran et al., https://arxiv.org/abs/1710.05941v1) where the SiLU was experimented with
     later.
     """
-    return x * torch.sigmoid(x)
 
+    def __init__(self):
+        if version.parse(torch.__version__) < version.parse("1.7"):
+            self.act = self._silu_python
+        else:
+            self.act = nn.functional.silu
 
-if version.parse(torch.__version__) < version.parse("1.7"):
-    silu = _silu_python
-else:
-    silu = nn.functional.silu
+    def _silu_python(self, input: Tensor) -> Tensor:
+        return input * torch.sigmoid(input)
 
+    def forward(self, input: Tensor) -> Tensor:
+        return self.act(input)
 
-def _mish_python(x):
+
+class MishActivation(nn.Module):
     """
     See Mish: A Self-Regularized Non-Monotonic Activation Function (Misra., https://arxiv.org/abs/1908.08681). Also
     visit the official repository for the paper: https://github.com/digantamisra98/Mish
     """
-    return x * torch.tanh(nn.functional.softplus(x))
 
+    def __init__(self):
+        super().__init__()
+        if version.parse(torch.__version__) < version.parse("1.9"):
+            self.act = self._mish_python
+        else:
+            self.act = nn.functional.mish
 
-if version.parse(torch.__version__) < version.parse("1.9"):
-    mish = _mish_python
-else:
-    mish = nn.functional.mish
+    def _mish_python(self, input: Tensor) -> Tensor:
+        return input * torch.tanh(nn.functional.softplus(input))
 
+    def forward(self, input: Tensor) -> Tensor:
+        return self.act(input)
 
-def linear_act(x):
-    return x
+
+class LinearActivation(nn.Module):
+    """
+    Applies the linear activation function, i.e. forwarding input directly to output.
+    """
+
+    def __init__(self):
+        super().__init__()
+
+    def forward(self, input: Tensor) -> Tensor:
+        return input
 
 
 ACT2FN = {
-    "relu": nn.functional.relu,
-    "silu": silu,
-    "swish": silu,
-    "gelu": gelu,
-    "tanh": torch.tanh,
-    "gelu_python": gelu_python,
-    "gelu_new": gelu_new,
-    "gelu_fast": gelu_fast,
-    "quick_gelu": quick_gelu,
-    "mish": mish,
-    "linear": linear_act,
-    "sigmoid": torch.sigmoid,
+    "relu": nn.ReLU(),
+    "silu": SiLUActivation(),
+    "swish": SiLUActivation(),
+    "gelu": GELUActivation(),
+    "tanh": nn.Tanh(),
+    "gelu_python": GELUActivation(use_gelu_python=True),
+    "gelu_new": NewGELUActivation(),
+    "gelu_fast": FastGELUActivation(),
+    "quick_gelu": QuickGELUActivation(),
+    "mish": MishActivation(),
+    "linear": LinearActivation(),
+    "sigmoid": nn.Sigmoid(),
 }
 
 
@@ -112,3 +158,14 @@ def get_activation(activation_string):
         return ACT2FN[activation_string]
     else:
         raise KeyError(f"function {activation_string} not found in ACT2FN mapping {list(ACT2FN.keys())}")
+
+
+# For backwards compatibility with: from activations import gelu_python
+gelu_python = get_activation("gelu_python")
+gelu_new = get_activation("gelu_new")
+gelu = get_activation("gelu")
+gelu_fast = get_activation("gelu_fast")
+quick_gelu = get_activation("quick_gelu")
+silu = get_activation("silu")
+mish = get_activation("mish")
+linear_act = get_activation("linear")

tests/test_activations.py

@@ -40,6 +40,10 @@ class TestActivations(unittest.TestCase):
         get_activation("gelu_new")
         get_activation("gelu_fast")
         get_activation("gelu_python")
+        get_activation("quick_gelu")
+        get_activation("mish")
+        get_activation("linear")
+        get_activation("sigmoid")
         with self.assertRaises(KeyError):
             get_activation("bogus")
         with self.assertRaises(KeyError):