Add FNetClassifier

[abheesht17]

This layer will be similar to BertClassifier.

Make sure you get all the layers, dropouts, etc. correct: https://github.com/google-research/google-research/blob/master/f_net/models.py#L358.

Wait for #643 and #646 to get merged.

[ADITYADAS1999]

Can I work with this issue after #643 and #646 get merged?

[durganshyadav]

I would like to work on this issue please assign me this issue.

[mattdangerw]

@ADITYADAS1999 assigned you!

@durganshyadav we will be continuing to open up contributions welcome issues each week. Since this is your first time contributing, #656 might be good as something really simple to get your feet wet? There is also #657 if you want something more involved.

[ADITYADAS1999]

This layer will be similar to BertClassifier.

Make sure you get all the layers, dropouts, etc. correct: https://github.com/google-research/google-research/blob/master/f_net/models.py#L358.

Wait for #643 and #646 to get merged.

So in this issue we have to add an python script named as f_net_classifier.py at keras-nlp/keras_nlp/models/f_net/
and this python script is similar to BertClassifier.