quant docs: add and clean up ELU (#40377)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40377

Cleans up the docstring for quantized ELU and adds it to the quantization docs.

Test Plan: * build on Mac OS and inspect

Differential Revision: D22162834

Pulled By: vkuzo

fbshipit-source-id: e548fd4dc8d67db27ed19cac4dbdf2a942586759
diff --git a/docs/source/quantization.rst b/docs/source/quantization.rst
index db94bca..870f19a 100644
--- a/docs/source/quantization.rst
+++ b/docs/source/quantization.rst
@@ -189,6 +189,7 @@
 
 * :meth:`~torch.nn.functional.relu` — Rectified linear unit (copy)
 * :meth:`~torch.nn.functional.relu_` — Rectified linear unit (inplace)
+* :meth:`~torch.nn.functional.elu` - ELU
 * :meth:`~torch.nn.functional.max_pool2d` - Maximum pooling
 * :meth:`~torch.nn.functional.adaptive_avg_pool2d` - Adaptive average pooling
 * :meth:`~torch.nn.functional.avg_pool2d` - Average pooling
@@ -353,6 +354,7 @@
 * :class:`~torch.nn.quantized.ReLU` — Rectified linear unit
 * :class:`~torch.nn.quantized.ReLU6` — Rectified linear unit with cut-off at
   quantized representation of 6
+* :class:`~torch.nn.quantized.ELU` — ELU
 * :class:`~torch.nn.quantized.Hardswish` — Hardswish
 * :class:`~torch.nn.quantized.BatchNorm2d` — BatchNorm2d. *Note: this module is usually fused with Conv or Linear. Performance on ARM is not optimized*.
 * :class:`~torch.nn.quantized.BatchNorm3d` — BatchNorm3d. *Note: this module is usually fused with Conv or Linear. Performance on ARM is not optimized*.
@@ -386,6 +388,7 @@
 * :func:`~torch.nn.quantized.functional.linear` — Linear (fully-connected) op
 * :func:`~torch.nn.quantized.functional.max_pool2d` — 2D max pooling
 * :func:`~torch.nn.quantized.functional.relu` — Rectified linear unit
+* :func:`~torch.nn.quantized.functional.elu` — ELU
 * :func:`~torch.nn.quantized.functional.hardsigmoid` — Hardsigmoid
 * :func:`~torch.nn.quantized.functional.hardswish` — Hardswish
 * :func:`~torch.nn.quantized.functional.hardtanh` — Hardtanh
@@ -749,6 +752,11 @@
 .. autoclass:: ReLU6
     :members:
 
+ELU
+~~~~~~~~~~~~~~~
+.. autoclass:: ELU
+    :members:
+
 Hardswish
 ~~~~~~~~~~~~~~~
 .. autoclass:: Hardswish
diff --git a/torch/nn/quantized/functional.py b/torch/nn/quantized/functional.py
index be66a98..ac93e49 100644
--- a/torch/nn/quantized/functional.py
+++ b/torch/nn/quantized/functional.py
@@ -455,16 +455,13 @@
 
 def elu(input, scale, zero_point, alpha=1.):
     # type: (Tensor, float, int, float) -> Tensor
-    r"""
-    Applies the quantized ELU function element-wise:
-
-    .. math::
-        \text{ELU}(x) = \max(0,x) + \min(0, \alpha * (\exp(x) - 1))
+    r"""This is the quantized version of :func:`~torch.nn.functional.elu`.
 
     Args:
         input: quantized input
-        scale, zero_point: Scale and zero point of the output tensor.
-        alpha: the :math:`\alpha` value for the ELU formulation. Default: 1.0
+        scale: quantization scale of the output tensor
+        zero_point: quantization zero point of the output tensor
+        alpha: the alpha constant
     """
     if not input.is_quantized:
         raise ValueError("Input to 'quantized.elu' must be quantized!")
diff --git a/torch/nn/quantized/modules/activation.py b/torch/nn/quantized/modules/activation.py
index e0fac03..1df7c06 100644
--- a/torch/nn/quantized/modules/activation.py
+++ b/torch/nn/quantized/modules/activation.py
@@ -106,7 +106,12 @@
         return Hardswish(float(scale), int(zero_point))
 
 class ELU(torch.nn.ELU):
-    r"""This is the quantized equivalent of :class:`torch.nn.ELU`.
+    r"""This is the quantized equivalent of :class:`~torch.nn.ELU`.
+
+    Args:
+        scale: quantization scale of the output tensor
+        zero_point: quantization zero point of the output tensor
+        alpha: the alpha constant
     """
     def __init__(self, scale, zero_point, alpha=1.):
         super(ELU, self).__init__(alpha)