Added sigmoid activation, and ability to do activation above linear layers.

manim_ml/neural_network/activation_functions/sigmoid.py

@@ -0,0 +1,15 @@
+from manim import *
+import numpy as np
+
+from manim_ml.neural_network.activation_functions.activation_function import (
+    ActivationFunction,
+)
+
+class SigmoidFunction(ActivationFunction):
+    """Sigmoid Activation Function"""
+
+    def __init__(self, function_name="Sigmoid", x_range=[-5, 5], y_range=[0, 1]):
+        super().__init__(function_name, x_range, y_range)
+
+    def apply_function(self, x_val):
+        return 1 / (1 + np.exp(-1 * x_val))