Modified some functions in C4W3 CarDetection file to make Tensorflow v2 compatible.

[Raghav4138]

modified some functions to be compatible with
TensorFlow version2.
The following file was modified.
".\coursera-deep-learning-specialization\C4 - Convolutional Neural Networks\Week 3\Car detection for Autonomous Driving\Autonomous_driving_application_Car_detection_v3a.ipynb"

DESCRIPTION:

General changes:
---------tf.function() -> tf.compat.v1.function()	

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In function yolo_filter_boxes()
    K.argmax -> tf.argmax
    K.reduce_max -> tf.reduce_max
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In function iou()
    used better names, to reduce code redundancy
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In function yolo_non_max_suppression()
---------these 2 lines were changed
    max_boxes_tensor = K.variable(max_boxes, dtype='int32')     # tensor to be used in tf.image.non_max_suppression()
    K.get_session().run(tf.variables_initializer([max_boxes_tensor])) # initialize variable max_boxes_tensor

---------new lines
    max_boxes_tensor = tf.keras.backend.variable(max_boxes, dtype='int32')     # tensor to be used in tf.image.non_max_suppression()
    with tf.compat.v1.Session() as sess:
        sess.run(tf.compat.v1.variables_initializer([max_boxes_tensor])) # initialize variable max_boxes_tensor

--------- K.gather()  ->  tf.gather()
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