Fix activation lookup with Python 3.12.3 (explosion#375)

We used the metaclass `EnumMeta`/`EnumType` to override reporting of
missing enum values (to give the full set of supported activations).
However, in Python 3.12.3, the default value of the `name` parameter of
`EnumType.__call__` method was changed from `None` to `_not_given`:

python/cpython@d771729

Even though this is a public API (which now uses a private default
value), it seems too risky to continue using it. So in this change, we
implement `Enum.__mising__` instead for the improved error reporting.

curated_transformers/layers/activations.py

@@ -1,52 +1,13 @@
 import math
-from enum import Enum, EnumMeta
+from enum import Enum
 from typing import Type
 
 import torch
 from torch import Tensor
 from torch.nn import Module
 
 
-class _ActivationMeta(EnumMeta):
- """
- ``Enum`` metaclass to override the class ``__call__`` method with a more
- fine-grained exception for unknown activation functions.
- """
-
- def __call__(
- cls,
- value,
- names=None,
- *,
- module=None,
- qualname=None,
- type=None,
- start=1,
- ):
- # Wrap superclass __call__ to give a nicer error message when
- # an unknown activation is used.
- if names is None:
- try:
- return EnumMeta.__call__(
- cls,
- value,
- names,
- module=module,
- qualname=qualname,
- type=type,
- start=start,
- )
- except ValueError:
- supported_activations = ", ".join(sorted(v.value for v in cls))
- raise ValueError(
- f"Invalid activation function `{value}`. "
- f"Supported functions: {supported_activations}"
- )
- else:
- return EnumMeta.__call__(cls, value, names, module, qualname, type, start)
-
-
-class Activation(Enum, metaclass=_ActivationMeta):
+class Activation(Enum):
  """
  Activation functions.
 
@@ -71,6 +32,14 @@ class Activation(Enum, metaclass=_ActivationMeta):
  #: Sigmoid Linear Unit (`Hendrycks et al., 2016`_).
  SiLU = "silu"
 
+ @classmethod
+ def _missing_(cls, value):
+ supported_activations = ", ".join(sorted(v.value for v in cls))
+ raise ValueError(
+ f"Invalid activation function `{value}`. "
+ f"Supported functions: {supported_activations}"
+ )
+
  @property
  def module(self) -> Type[torch.nn.Module]:
  """