[Relay][Documentation] Behavior of relay.stack does not appear to match documentation

[slyubomirsky]

The documentation for tvm.relay.stack states that its input can be either a list of Relay expressions or a tuple. However, if you give a Relay expression with a tuple output as an input to tvm.relay.stack, this will result in a Python type error, as the function first tries to coerce the input into a Python list. The functionality should either be made to match the documentation or the documentation should be corrected to reflect the intended use.

Edit: Based on test_stack, it appears to work with tuple literals but not expressions that evaluate to tuples. I will create a specific test case to demonstrate this issue.

For example, the following will result in a Python type error:

    x = relay.Tuple([relay.const(np.random.rand(1, 1)), relay.const(np.random.rand(1, 1))])
    y = relay.Var('y')
    l = relay.Let(y, x, y)
    z = relay.stack(l, axis=1)

@hogepodge

[slyubomirsky]

It appears this can be easily fixed with some changes to the Python binding for stack. I will PR a fix. (See #7130)

[slyubomirsky]

In retrospect, it is conceivable that the documentation did correctly note that the input must be a list or a tuple literal, but I would argue that restricting the possible input in this way defies user expectations. Also the fix was very easy.