'Signal the start of a block of repeated computation (at the start
of a loop).  This operation can be used to help the compiler optimize
instruction performance, but has no direct effect on calculations.
It must be book-ended by a corresponding Backend.end() call.
Note that multiple begin calls can appear adjacent in nested loops.
Arguments:
block (Block.attr): identifies the type of computation being worked
on based on Block attribute specified
identifier (int): unique identifier for this particular iteration
of the block.  Will typically be something like
epoch number, mini-batch number, and so forth.
See Also:
:py:func:`~neon.backends.backend.Backend.end`'

def begin(self, block, identifier):

    pass
