exp2 extension

[nassimberrada]

Close #6196

[Ishticode]

About the tensorflow backend
I am not fully sure of the consequences of np_config.enable_numpy_behavior().... But enabling it file wide could lead to some unforeseen results in other functions.

I think its best if we use

    return tf.math.pow(2, x)

instead of involving numpy into this at all.

So could you delete the np config import and behaviour setting and updte the implementation in the tesorflow backend.

[nassimberrada]

I actually got an error that suggested using that when making use of numpy related functions, unless i used it the tests wouldn't pass. Switched for tf.math.pow as you suggested and it works as well, that's definitely better than relying on a file-wide change.

[Ishticode]

We actually dont need the tf.cast. It should be removed.
It should just be return tf.math.pow(2,x). I think it should pass. Lmk when done and hopefully its ready

And the error for numpy related behaviour wast that tf's numpy experimental exp2 called x.astype internally with x being a tf tensor. This required np behaviour to be enabled. Bus fortunately we can steer clear from the whole thing.

[nassimberrada]

I see, yeah the error was definitely related to the "astype" element, makes sense.
Removed tf.cast.