Metal backend is kind of broken for external arrays args

[k-ye]

The size of an array arg on Metal is still determined at compile-time, and is fixed after JIT compilation. See https://github.com/taichi-dev/taichi/blob/6d538e171c96193fd2ce27967bd0b17a5797565e/taichi/backends/metal/

With the introduction of Ndarray (ti.ndarray), array size is a runtime information. This means that arrays of different size won't re-trigger a kernel JIT. Hence, a Metal kernel with external arrays is now broken, if it's invoked twice with arrays of different size.

The only reason Metal backend still seems work when it's first invoked with an array is reasoned below:

1. Kernel::Arg::size is zero at arg declaration time:

   taichi/taichi/program/callable.cpp

   Lines 16 to 26 in 6d538e1

   	int Callable::insert_ret(const DataType &dt) {
   	rets.emplace_back(dt->get_compute_type());
   	return (int)rets.size() - 1;
   	}
   	int Callable::insert_arr_arg(const DataType &dt,
   	int total_dim,
   	std::vector<int> element_shape) {
   	args.emplace_back(dt->get_compute_type(), true, /*size=*/0, total_dim,
   	element_shape);
   	return (int)args.size() - 1;
   	}

2. When setting the external array arg at

   taichi/python/taichi/lang/kernel_impl.py

   Lines 667 to 669 in 6d538e1

   	launch_ctx.set_arg_external_array_with_shape(
   	actual_argument_slot, int(tmp.ctypes.data),
   	tmp.nbytes, v.shape)

   , it eventually calls into here:

   taichi/taichi/program/kernel.cpp

   Line 241 in 6d538e1

   kernel_->args[arg_id].size = size;

   . Note that Kernel::Arg::size is changed at runtime!
3. Then the kernel is invoked via Kernel::operator()

   taichi/taichi/program/kernel.cpp

   Lines 106 to 108 in 6d538e1

   	if (!compiled_) {
   	compile();
   	}

   . At this point, Metal's codegen starts working. By the time it gets to

   taichi/taichi/backends/metal/kernel_utils.cpp

   Line 91 in 6d538e1

   ma.stride = ma.is_array ? ka.size : dt_bytes;

   , because of step 2, the array size has been set correctly.

---

We should do two things:

1. Stop doing this:

   taichi/taichi/program/kernel.cpp

   Line 241 in 6d538e1

   kernel_->args[arg_id].size = size;

   . The runtime size should be set in

   taichi/taichi/program/context.h

   Line 16 in 6d538e1

   struct RuntimeContext {

   .
2. Change Metal backend's implementaion for array args. Instead of embedding a fixed size of memory within the Context buffer, we should follow other backends' Ndarray implementation. WIP in [Metal] Support Ndarray #4720

[k-ye]

Fixed by #4720