How to support Python build-in functions like len()?

## ❓How to support Python build-in functions like len()?

## System Information
- pytorch: 1.9.1
- coremltools: 1.5.1

I am trying to convert my model in Core ML with Coremltools. I had some issue that I replicated in this simple example.

```python

import torch
import torch.nn as nn
import coremltools

class SimpleTest(nn.Module):
def forward(self, x):
value = len(x)
return torch.tensor(value)

simple_model = SimpleTest()

scripted_model = torch.jit.script(simple_model)
mlmodel = coremltools.converters.convert(scripted_model, inputs=[coremltools.TensorType(shape=(400,))], debug=True)

```

I have this error.

*debug=True info*

```txt
the following model ops are IMPLEMENTED:
constant
tensor
the following model ops are MISSING:
len
```

How can I use Python **len()** funcion inside the model?

Start with the SimpleTest.forward reproduction and the coremltools.converters.convert call, then inspect the converter's debug report showing len as missing. Determine where supported model operations are registered and how similar operations are tested. Done means the example converts successfully with len(x) and the resulting model behaves as expected.

Written by the indexing model from the issue text.