!10519 update documentation of warmup_lr, F1, RMSProp, Batchnorm2d and add some pictures of links of activation function.

From: @wangshuide2020
Reviewed-by: @liangchenghui,@wuxuejian
Signed-off-by: @liangchenghui

mindspore/nn/dynamic_lr.py

@@ -343,13 +343,12 @@ def warmup_lr(learning_rate, total_step, step_per_epoch, warmup_epoch):
 
     Args:
         learning_rate (float): The initial value of learning rate.
-        warmup_steps (int): The warm up steps of learning rate.
-
-    Inputs:
-        Tensor. The current step number.
+        total_step (int): The total number of steps.
+        step_per_epoch (int): The number of steps in per epoch.
+        warmup_epoch (int): A value that determines the epochs of the learning rate is warmed up.
 
     Returns:
-        Tensor. The learning rate value for the current step.
+        list[float]. The size of list is `total_step`.
 
     Examples:
         >>> learning_rate = 0.1

mindspore/nn/layer/activation.py

@@ -142,6 +142,9 @@ class ELU(Cell):
         \text{alpha} * (\exp(x_i) - 1), &\text{otherwise.}
         \end{cases}
 
+    The picture about ELU looks like this `ELU <https://en.wikipedia.org/wiki/
+    Activation_function#/media/File:Activation_elu.svg>`_.
+
     Args:
         alpha (float): The coefficient of negative factor whose type is float. Default: 1.0.
 
@@ -178,6 +181,9 @@ class ReLU(Cell):
     element-wise :math:`\max(0, x)`, specially, the neurons with the negative output
     will be suppressed and the active neurons will stay the same.
 
+    The picture about ReLU looks like this `ReLU <https://en.wikipedia.org/wiki/
+    Activation_function#/media/File:Activation_rectified_linear.svg>`_.
+
     Inputs:
         - **input_data** (Tensor) - The input of ReLU.
 
@@ -335,6 +341,9 @@ class GELU(Cell):
     :math:`GELU(x_i) = x_i*P(X < x_i)`, where :math:`P` is the cumulative distribution function
     of standard Gaussian distribution and :math:`x_i` is the element of the input.
 
+    The picture about GELU looks like this `GELU <https://en.wikipedia.org/wiki/
+    Activation_function#/media/File:Activation_gelu.png>`_.
+
     Inputs:
         - **input_data** (Tensor) - The input of GELU.
 
@@ -410,6 +419,9 @@ class Sigmoid(Cell):
     Sigmoid function is defined as:
     :math:`\text{sigmoid}(x_i) = \frac{1}{1 + \exp(-x_i)}`,    where :math:`x_i` is the element of the input.
 
+    The picture about Sigmoid looks like this `Sigmoid <https://en.wikipedia.org/wiki/
+    Sigmoid_function#/media/File:Logistic-curve.svg>`_.
+
     Inputs:
         - **input_data** (Tensor) - The input of Tanh.
 
@@ -448,6 +460,9 @@ class PReLU(Cell):
     Parameter :math:`w` has dimensionality of the argument channel. If called without argument
     channel, a single parameter :math:`w` will be shared across all channels.
 
+    The picture about PReLU looks like this `PReLU <https://en.wikipedia.org/wiki/
+    Activation_function#/media/File:Activation_prelu.svg>`_.
+
     Args:
         channel (int): The dimension of input. Default: 1.
         w (float): The initial value of w. Default: 0.25.

mindspore/nn/layer/normalization.py

@@ -340,6 +340,9 @@ class BatchNorm2d(_BatchNorm):
     Note:
         The implementation of BatchNorm is different in graph mode and pynative mode, therefore that mode can not be
         changed after net was initilized.
+        Note that the formula for updating the running_mean and running_var is
+        :math:`\hat{x}_\text{new} = (1 - \text{momentum}) \times x_t + \text{momentum} \times \hat{x}`,
+        where :math:`\hat{x}` is the estimated statistic and :math:`x_t` is the new observed value.
 
     Args:
         num_features (int): `C` from an expected input of size (N, C, H, W).

mindspore/nn/metrics/fbeta.py

@@ -122,7 +122,7 @@ class Fbeta(Metric):
 class F1(Fbeta):
     r"""
     Calculates the F1 score. F1 is a special case of Fbeta when beta is 1.
-    Refer to class `Fbeta` for more details.
+    Refer to class :class: `mindspore.nn.Fbeta` for more details.
 
     .. math::
         F_1=\frac{2\cdot true\_positive}{2\cdot true\_positive + false\_negative + false\_positive}

mindspore/nn/optim/rmsprop.py

@@ -42,13 +42,6 @@ class RMSProp(Optimizer):
     """
     Implements Root Mean Squared Propagation (RMSProp) algorithm.
 
-    Note:
-        When separating parameter groups, the weight decay in each group will be applied on the parameters if the
-        weight decay is positive. When not separating parameter groups, the `weight_decay` in the API will be applied
-        on the parameters without 'beta' or 'gamma' in their names if `weight_decay` is positive.
-
-        To improve parameter groups performance, the customized order of parameters can be supported.
-
     Update `params` according to the RMSProp algorithm.
 
     The equation is as follows:
@@ -88,6 +81,13 @@ class RMSProp(Optimizer):
     :math:`\\eta` is learning rate, represents `learning_rate`. :math:`\\nabla Q_{i}(w)` is gradientse,
     represents `gradients`.
 
+    Note:
+        When separating parameter groups, the weight decay in each group will be applied on the parameters if the
+        weight decay is positive. When not separating parameter groups, the `weight_decay` in the API will be applied
+        on the parameters without 'beta' or 'gamma' in their names if `weight_decay` is positive.
+
+        To improve parameter groups performance, the customized order of parameters can be supported.
+
     Args:
         params (Union[list[Parameter], list[dict]]): When the `params` is a list of `Parameter` which will be updated,
             the element in `params` must be class `Parameter`. When the `params` is a list of `dict`, the "params",