add support for trace function

[SigureMo]

#2 的一个 TODO，尝试简单支持了下 Callable（仅支持函数，不支持显式重载 __call__ 的 Callable object，试了下 torch.fx 也是不支持的），keyword args 还没支持～

> python examples/simple_callable_trace.py

python IR:
opcode         name    target                          args      kwargs
-------------  ------  ------------------------------  --------  --------------------
placeholder    x       x                               ()        {}
placeholder    y       y                               ()        {}
call_function  mul     <built-in function mul>         (x, x)    {}
call_function  add     <built-in function add>         (mul, y)  {}
call_function  add     <function add at 0x13fc54430>   ()        {'x': add, 'y': add}
call_function  relu    <function relu at 0x16c09b2e0>  ()        {'x': add}
output         output  output                          (relu,)   {}
python code generated:
mul = x * x
add = mul + y
add = paddle.tensor.math.add(x = add, y = add)
relu = paddle.nn.functional.activation.relu(x = add)
return (relu,)

[gglin001]

lgtm

[gglin001]

I have intived you as the admin for this repo, please check your email @SigureMo