feat: add Type hints for examples/keras_api_example.py

[Neerajpathak07]

Progresses #124

This PR adds a type hint for the examples file keras_api_example.py.
Optimizing the function from:-

def get_model():

def load_data():

to:-

def get_model() -> keras.Model:
  """Builds and returns the CNN model."""

def load_data() -> Tuple[
    Tuple[np.ndarray, np.ndarray],
    Tuple[np.ndarray, np.ndarray],
]: