Better error messaging for requiring ModuleList to be constant

[sidazhang]

🚀 Feature

def __init__
        // ....
        self.conv_lstms = nn.ModuleList(conv_lstms)
        // ...

def forward()
        for i in range(len(self.cell_n)):
            cell = self.conv_lstms[i]

python value of type 'ModuleList' cannot be used as a value:
def forward(self, inputs, hidden):
    for i in range(len(self.cell_n)):
        cell = self.conv_lstms[i]
               ~~~~~~~~~ <--- HERE
        hidden[i] = cell(step_input, hidden[i])
        step_input = hidden[i][0]

    return step_input, hidden

The fix in the above case is to declare __constants__ = ["conv_lstms"]

So the error message should say that ModuleList should be declared constant

Motivation

The error message is very unclear for new users.

[suo]

Thanks for the feedback! We will look to improve our error messaging in the next release. This issue has been added to our internal triage.

[sto-chastic]

My code (follows) appends to ModuleList, therefore when I declare ModuleList as constant, I get the error:

AttributeError: _ConstModuleList object has no attribute append

Is there a work arround for this?
Code:

`class resblock(ScriptModule):
constants = ["module_list"]

def __init__(self, ch, nblocks=1, shortcut=True):

    super().__init__()
    self.shortcut = shortcut
    self.module_list = nn.ModuleList()
    for i in range(nblocks):
        resblock_one = nn.ModuleList()
        resblock_one.append(add_conv(ch, ch//2, 1, 1))
        resblock_one.append(add_conv(ch//2, ch, 3, 1))
        self.module_list.append(resblock_one)

@script_method
def forward(self, x):
    for i in range(len(self.module_list)):
        module = self.module_list[i]
        h = x
        for res in module:
            h = res(h)
        x = x + h if self.shortcut else h
    return x`

[sidazhang]

You need to append the resnet blocks into an array and then initialize the module_list

For example

m = []
m.append(blah)
self.module_list = nn.ModuleList(m)

[eellison]

#17167 suggests add to __constants__ when a module list is used as a tuple