[bug] render_model cannot show ∈ symbol in the saved file

[dilaragokay]

Issue Description

I created a simple model and rendered it the following way in Colab.

import pyro
import torch
from pyro.distributions import constraints, Uniform, LogNormal

def model(data):
    low = torch.tensor(0.)
    high = torch.tensor(5.)
    z = pyro.sample("difficulty", Uniform(low, high))

    sigma_grades = pyro.param("sigma_grades", torch.tensor(10.), constraint = constraints.positive)
    with pyro.plate("data", data.size(0)):
        pyro.sample("grades", LogNormal(90 - 10 * z, sigma_grades), obs = data)

data = torch.rand(70) * 100

pyro.render_model(model, model_args=(data,), filename='my_render.png',render_distributions=True, render_params=True)

I see in the notebook that the file is rendered the following way

The above image is from a screenshot. When I download my_render.png and open the file, I see that the ∈ symbol is not shown correctly.

Edit 1: I have just tried with a few more file extensions. Saving as an SVG works perfectly. Saving as a PDF does not work either.
Edit 2: This might be a GraphViz issue actually: https://stackoverflow.com/questions/55456467/how-to-include-element-symbol-in-graphviz

Environment

I am running this experiment on Colab. I installed Pyro in the first cell with !pip3 install pyro-ppl.

• Python version: 3.7.13
• PyTorch version: 1.11.0+cu113
• Pyro version: 1.8.1

[fehiepsi]

I guess we can add a note in the docs that it is better to use svg format when render_params=True.

[fritzo]

we could just use a : symbol as in type theory and avoid the ∈ issue

[eb8680]

Should be addressed by #3097