[Lang] Support the functions of dataclass as kernel argument and retu…

…rn value (#7865)

Issue: fixes #7822

<!--
copilot:all
-->
### <samp>🤖 Generated by Copilot at 4ae87d7</samp>

### Summary
🧪🛠️🚀

<!--
1. 🧪 - This emoji represents testing, experimentation, or science, and
can be used to indicate the addition of test cases or the verification
of functionality.
2. 🛠️ - This emoji represents tools, construction, or repair, and can be
used to indicate the addition of a new feature or the improvement of an
existing one.
3. 🚀 - This emoji represents speed, launch, or innovation, and can be
used to indicate the enhancement of performance, the expansion of
capabilities, or the introduction of a novel idea.
-->
This pull request enables data classes defined by `ti.dataclass` to have
methods decorated by `ti.func` and `ti.kernel`. It also adds tests to
check the functionality and correctness of this feature.

> _`StructType` creates the data class of doom_
> _With `__struct_methods` to unleash its power_
> _Call them from Python, or pass them to `ti.kernel`_
> _`ti.func` returns them, the ultimate metal_

### Walkthrough
* Add `__struct_methods` attribute to data class dictionaries to enable
access of methods defined in `ti.dataclass` decorator
([link](https://github.com/taichi-dev/taichi/pull/7865/files?diff=unified&w=0#diff-3154e0533b9fd63e663c16c2e87c65068c3b002a65cf80529b6577d173bdb5feR633),
[link](https://github.com/taichi-dev/taichi/pull/7865/files?diff=unified&w=0#diff-3154e0533b9fd63e663c16c2e87c65068c3b002a65cf80529b6577d173bdb5feR654))
* Add test cases for using `ti.func` and `ti.kernel` as methods of data
classes, both as kernel arguments and kernel returns, in
`test_struct.py`
([link](https://github.com/taichi-dev/taichi/pull/7865/files?diff=unified&w=0#diff-e87bf5cb1cd09e10b5cfa001ab2ef18f31a242db3a7b66ee98a76d60b1615e71R79-R130))

python/taichi/lang/struct.py

@@ -630,6 +630,7 @@ def from_taichi_object(self, func_ret, ret_index=()):
                 d[name] = dtype.from_taichi_object(func_ret, ret_index + (index,))
             else:
                 d[name] = expr.Expr(_ti_core.make_get_element_expr(func_ret.ptr, ret_index + (index,)))
+        d["__struct_methods"] = self.methods
 
         return Struct(d)
 
@@ -650,6 +651,7 @@ def from_kernel_struct_ret(self, launch_ctx, ret_index=()):
                     d[name] = launch_ctx.get_struct_ret_float(ret_index + (index,))
                 else:
                     raise TaichiRuntimeTypeError(f"Invalid return type on index={ret_index + (index, )}")
+        d["__struct_methods"] = self.methods
 
         return Struct(d)
 

tests/python/test_struct.py

@@ -76,3 +76,55 @@ def test_2d_nested():
     for i in range(n * 2):
         for j in range(n):
             assert x[i, j] == i + j * 10
+
+
+@test_utils.test()
+def test_func_of_data_class_as_kernel_arg():
+    @ti.dataclass
+    class Foo:
+        x: ti.f32
+        y: ti.f32
+
+        @ti.func
+        def add(self, other: ti.template()):
+            return Foo(self.x + other.x, self.y + other.y)
+
+    @ti.kernel
+    def foo_x(x: Foo) -> ti.f32:
+        return x.add(x).x
+
+    assert foo_x(Foo(1, 2)) == 2
+
+    @ti.kernel
+    def foo_y(x: Foo) -> ti.f32:
+        return x.add(x).y
+
+    assert foo_y(Foo(1, 2)) == 4
+
+
+@test_utils.test(arch=[ti.cpu, ti.cuda, ti.amdgpu])
+def test_func_of_data_class_as_kernel_return():
+    # TODO: enable this test in SPIR-V based backends after SPIR-V based backends can return structs.
+    @ti.dataclass
+    class Foo:
+        x: ti.f32
+        y: ti.f32
+
+        @ti.func
+        def add(self, other: ti.template()):
+            return Foo(self.x + other.x, self.y + other.y)
+
+        def add_python(self, other):
+            return Foo(self.x + other.x, self.y + other.y)
+
+    @ti.kernel
+    def foo(x: Foo) -> Foo:
+        return x.add(x)
+
+    b = foo(Foo(1, 2))
+    assert b.x == 2
+    assert b.y == 4
+
+    c = b.add_python(b)
+    assert c.x == 4
+    assert c.y == 8