[BUG] Customize get Python AST starting_line_no is a null ptr when isinstance(fn, str)

[jiahanxie353]

Describe the bug
If we take the if branch when getting the Python AST in customize, then starting_line_no is a null pointer in this line, causing:

UnboundLocalError: cannot access local variable 'starting_line_no' where it is not associated with a value

To Reproduce

import torch
import torch.nn as nn
import torch.nn.functional as F

import allo

torch.set_printoptions(precision=2)

input_size = 6
hidden_size = 5
output_size = 2

mean = 0
std = 5

lower_bound = -5
upper_bound = 5

torch.manual_seed(42)

random_tensor = torch.randn(hidden_size, input_size) * std + mean

clipped_tensor = torch.clamp(random_tensor, min=lower_bound, max=upper_bound)


class Model(nn.Module):
    def __init__(self, input_size, hidden_size, output_size):
        super(Model, self).__init__()
        self.l1 = nn.Linear(input_size, hidden_size)
        self.activation = F.relu
        self.l2 = nn.Linear(hidden_size, output_size)

    def forward(self, x):
        output = self.l1(x)
        output = self.activation(output)
        output = self.l2(output)
        return output

model = Model(input_size, hidden_size, output_size)
model.eval()
example_inputs = [clipped_tensor]
mlir_mod = allo.frontend.from_pytorch(
    model, example_inputs=example_inputs, verbose=False
)
print(mlir_mod)

Buggy output

UnboundLocalError: cannot access local variable 'starting_line_no' where it is not associated with a value

Expected behavior
Expect to run successfully.