Bug/UX issue: `Variable("key")` misinterprets string as `input_variables`

[LucasBoTang]

Description

There is a positional argument misalignment in the Variable class. Initializing a Variable with a string (e.g., Variable("x")) causes the string to be assigned to input_variables instead of key.

Because strings are iterable, make_graph iterates over the characters, treating them as constants and creating an anonymous node. This leads to:

1. _is_input being False, making the variable unable to fetch data from a dictionary.
2. key being assigned a random object ID (e.g., '140...) instead of the intended string.

Example of unexpected behavior

from neuromancer.constraint import Variable

# User intends to create an input variable with key "x"
v = Variable("x")

print(f"Is input: {v._is_input}")  # Returns False, expected True
print(f"Key: {v.key}")                # Returns random ID, expected "x"