Outer product between vectors not work

[BillXu2000]

Describe the bug
Matrix multiplication between n1 and 1n vector will cause compilation error.

To Reproduce

import taichi as ti
ti.init()

@ti.kernel
def fun():
    a = ti.Vector([0.0, 0.0])
    print(a @ a.transpose())

fun()

Log/Screenshots

{15:04}~/playground:dev ✗ ➭ p test_ra.py
[Taichi] version 1.1.3, llvm 10.0.0, commit 9a35e049, linux, python 3.8.5
[Taichi] Starting on arch=x64
Traceback (most recent call last):
  File "test_ra.py", line 9, in <module>
    fun()
  File "/home/bx2k/git/taichi/python/taichi/lang/kernel_impl.py", line 917, in wrapped
    raise type(e)('\n' + str(e)) from None
taichi.lang.exception.TaichiCompilationError: 
File "test_ra.py", line 7, in fun:
    print(a @ a.transpose())
          ^^^^^^^^^^^^^^^^^
Traceback (most recent call last):
  File "/home/bx2k/git/taichi/python/taichi/lang/ast/ast_transformer_utils.py", line 25, in __call__
    return method(ctx, node)
  File "/home/bx2k/git/taichi/python/taichi/lang/ast/ast_transformer.py", line 738, in build_BinOp
    node.ptr = op(node.left.ptr, node.right.ptr)
  File "/home/bx2k/git/taichi/python/taichi/lang/ast/ast_transformer.py", line 736, in <lambda>
    ast.MatMult: lambda l, r: l @ r,
  File "/home/bx2k/git/taichi/python/taichi/lang/matrix.py", line 564, in __matmul__
    return other.transpose() @ self
  File "/home/bx2k/git/taichi/python/taichi/lang/matrix.py", line 566, in __matmul__
    assert self.m == other.n, f"Dimension mismatch between shapes ({self.n}, {self.m}), ({other.n}, {other.m})"
AssertionError: Dimension mismatch between shapes (2, 1), (2, 1)

Additional comments
This bug seems to be related to PR #6014 .

[strongoier]

cc: @AD1024

[AD1024]

Thanks! Just working on a PR