Add docstring to is_keras_tensor function
Hi @MadhumithaKolkar It looks like your PR relates to the Keras component. Please submit it to the github.com/keras-team/keras repository instead. Thank you for your contribution! 
@fchollet, @qlzh727
"This PR adds a missing docstring to the is_keras_tensor function in the Node class. The is_keras_tensor function is used to check whether an object is a Keras tensor by checking if it has a _keras_history attribute. However, it lacked a docstring explaining its purpose and usage. This change improves code readability and documentation clarity."