passing postitional args as kw for `paddle.Tensor.astype` at paddle frontend

[akshatvishu]

Currently the implementation for paddle.Tensor.astype is passing positional argument dtype as keyword argument which is throwing an error.

Implementation: code

Also, fixed the decorator to specify bfloat16 as the only supportive as ivy doesn't it at paddle backend yet, other than that all dtypes should be accepted by astype