Fix EinsumHelper::ParseEquation to avoid uninitialized accesses.

EinsumHelper::ParseEquation is supposed to return true or false in
input_has_ellipsis and output_has_ellipsis to indicate whether there is
ellipsis in the inputs and output. Previously, when there is no ellipsis in the
inputs or output, the routine doesn't assign false to the variables. This
change initializes the two variables with false to fix the problem.
PiperOrigin-RevId: 391772004
Change-Id: I17b6c88aadef4131470378e48cced054bf252e86