[XLA] Remove default argument on virtual function DeviceMemoryAllocator::Allocate().

Default args on virtual functions are disallowed by the Google style
guide, for good reason.  They have the extremely surprising behavior
that the defaults you get when calling a function on a pointer depend
not on the underlying type of the object, but on whatever is the
semantic type of the pointer!

PiperOrigin-RevId: 193611213