No proper error message for autodiff not supporting to_numpy and from_numpy

[erizmr]

Describe the bug
No proper error message for autodiff not supporting to_numpy and from_numpy.
The reason for this bug is that autodiff not supporting external arrary currently, the dest of a GlobalStoreStmt cannot be a ExternalPtrStmt , thus an assertion failure raised.

To Reproduce

import taichi as ti

ti.init()
a = ti.field(dtype=float, shape=(), needs_grad=True)
loss = ti.field(dtype=float, shape=(), needs_grad=True)

def func():
  b = a.to_numpy()
  print(b)

with ti.Tape(loss=loss):
  func()
	

Log/Screenshots

Additional comments
Related issue #4267 .

[erizmr]

Fixed by #5630.