derive COPY nbytes late in realize [pr] (tinygrad#8137)

* derive COPY arg later in realize [pr]

* can assume no implicit casts or movement ops here

tinygrad/engine/lazy.py

@@ -123,7 +123,7 @@ def const_arg(self) -> ConstType:
 
   def _copy(self, device:str) -> LazyBuffer:
     assert self.st.contiguous and self.size == self.base.size, f"can only copy contig {self} {self.base}"
-    return create_lazybuffer(device, ShapeTracker.from_shape(self.shape), self.dtype, Ops.COPY, self.buffer.nbytes, (self,), enable_cache=False)
+    return create_lazybuffer(device, ShapeTracker.from_shape(self.shape), self.dtype, Ops.COPY, srcs=(self,), enable_cache=False)
 
   def copy_to_device(self, device:str, force:bool=False, clone:bool=False) -> LazyBuffer:
     # no COPY

tinygrad/engine/realize.py

@@ -186,12 +186,12 @@ def lower_schedule_item(si:ScheduleItem) -> ExecItem:
   if si.ast.op is Ops.SINK:
     runner = get_runner(si.outputs[0].device, si.ast)
     return ExecItem(runner, [si.bufs[x] for x in runner.p.globals], si.metadata)
-  out, arg = si.outputs[0], si.ast.arg
+  out = si.outputs[0]
   if si.ast.op is Ops.COPY:
     kernel_type = BufferCopy
     if hasattr(Device[out.device].allocator, '_transfer') and out.device.split(":")[0] == si.inputs[0].device.split(":")[0]:
       kernel_type = BufferXfer
-    return ExecItem(kernel_type(arg, out.device, si.inputs[0].device), list(si.bufs))
+    return ExecItem(kernel_type(out.nbytes, out.device, si.inputs[0].device), list(si.bufs))
   if si.ast.op is Ops.EMPTY: return ExecItem(EmptyOp(out), list(si.bufs))
   if si.ast.op is Ops.BUFFER_VIEW: return ExecItem(ViewOp(out), list(si.bufs))
   raise RuntimeError(f"don't know how to lower {si.ast}")