Feature: FastAP Loss Function

[KarahanS]

• [x] FastAP Loss function will be implemented.
• [x] Test for the new loss function will be implemented.

Reference implementations:

• https://github.com/kunhe/FastAP-metric-learning/blob/master/pytorch/FastAP_loss.py
• https://github.com/KevinMusgrave/pytorch-metric-learning/blob/691a6354be130547a8e26e170b86cf65c36cd791/src/pytorch_metric_learning/losses/fast_ap_loss.py

[KarahanS]

Please refer to the PR #199.

[KarahanS]

• FastAP loss function is implemented.
• Unittest is prepared - in which we compare the result of our loss function with that of the official implementation.
• Loss function is implemented to comply with device consistency and tested using gpu.