Error in execution step -- "TypeError:can only concatenate str (not "int") to str"

Hi! Thanks for the great work :)

I was running the NLVR notebook and get the following error for the first two statements. Any idea why?

```---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In[15], line 1
----> 1 result, prog_state, html_str = interpreter.execute(prog,init_state,inspect=True)

File ~/visprog/engine/utils.py:38, in ProgramInterpreter.execute(self, prog, init_state, inspect)
36 for prog_step in prog_steps:
37 if inspect:
---> 38 step_output, step_html = self.execute_step(prog_step,inspect)
39 html_str += str(step_html) + '

---

'
40 else:

File ~/visprog/engine/utils.py:24, in ProgramInterpreter.execute_step(self, prog_step, inspect)
22 step_name = parse_step(prog_step.prog_str,partial=True)['step_name']
23 print(step_name)
---> 24 return self.step_interpreters[step_name].execute(prog_step,inspect)

File ~/visprog/engine/step_interpreters.py:103, in EvalInterpreter.execute(self, prog_step, inspect)
100 step_input = step_input.replace('xor','!=')
102 step_input = step_input.format(**prog_state)
--> 103 step_output = eval(step_input)
104 prog_step.state[output_var] = step_output
105 if inspect:

File :1

TypeError: can only concatenate str (not "int") to str
```

The program generated is as follows:
```
ANSWER0=VQA(image=LEFT,question='How many camels are in the image?')
ANSWER1=VQA(image=RIGHT,question='How many camels are in the image?')
ANSWER2=VQA(image=LEFT,question='How many people are in the image?')
ANSWER3=VQA(image=RIGHT,question='How many people are in the image?')
ANSWER4=EVAL(expr='{ANSWER0} + {ANSWER1} > {ANSWER2} + {ANSWER3}')
FINAL_ANSWER=RESULT(var=ANSWER4)
```