what tf.compat.v1.reset_default_graph() is actually doing ?

In TF1.x by default, session is constructed around the default graph. To clear the default graph stack and reset the global default graph you can use the tf.compat.v1.reset_default_graph function in TF2.x.

Note: The default graph is a property of the current thread. This function applies only to the current thread. Calling this function while a tf.compat.v1.Session or tf.compat.v1.InteractiveSession is active will result in undefined behavior. Using any previously created tf.Operation or tf.Tensor objects after calling this function will result in undefined behavior.