coremltools.converters.convert gives a RuntimeError during conversion to .mlmodel

## 🐞Describe the bug
We have followed the instructions in [Model Scripting](https://coremltools.readme.io/docs/model-scripting) to convert our custom PyTorch model to .mlmodel. Please find our google colab notebook [here](https://colab.research.google.com/drive/1Ha_-zvu0diDdlPnYr8RiEYdD80KBkO9k?usp=sharing)

## Stack Trace
```
[/usr/local/lib/python3.7/dist-packages/coremltools/converters/mil/frontend/torch/converter.py](https://localhost:8080/#) in _lower_graph_block(graph)
377 attr_name = getattr(node, node.kindOf("name"))("name")
378
--> 379 module = getattr(node_to_module_map[_input], attr_name)
380 node_to_module_map[_output] = module
381

KeyError: images.7 defined in (%images.7 : __torch__.torchvision.models.detection.image_list.ImageList, %targets.31 : Dict(str, Tensor)[]? = prim::TupleUnpack(%427)
)
```

## To Reproduce
Run our google colab notebook to [reproduce](https://colab.research.google.com/drive/1Ha_-zvu0diDdlPnYr8RiEYdD80KBkO9k?usp=sharing) the issue.

Model conversion fails in the above Google Colab notebook.

## System environment (please complete the following information):
PyTorch version: 1.11.0+cu113
Is debug build: False
CUDA used to build PyTorch: 11.3
ROCM used to build PyTorch: N/A

OS: Ubuntu 18.04.5 LTS (x86_64)
GCC version: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
Clang version: 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
CMake version: version 3.22.4
Libc version: glibc-2.26

Python version: 3.7.13 (default, Apr 24 2022, 01:04:09) [GCC 7.5.0] (64-bit runtime)
Python platform: Linux-5.4.188+-x86_64-with-Ubuntu-18.04-bionic
Is CUDA available: True
CUDA runtime version: 11.1.105
GPU models and configuration: GPU 0: Tesla T4
Nvidia driver version: 460.32.03
cuDNN version: Probably one of the following:
/usr/lib/x86_64-linux-gnu/libcudnn.so.7.6.5
/usr/lib/x86_64-linux-gnu/libcudnn.so.8.0.5
/usr/lib/x86_64-linux-gnu/libcudnn_adv_infer.so.8.0.5
/usr/lib/x86_64-linux-gnu/libcudnn_adv_train.so.8.0.5
/usr/lib/x86_64-linux-gnu/libcudnn_cnn_infer.so.8.0.5
/usr/lib/x86_64-linux-gnu/libcudnn_cnn_train.so.8.0.5
/usr/lib/x86_64-linux-gnu/libcudnn_ops_infer.so.8.0.5
/usr/lib/x86_64-linux-gnu/libcudnn_ops_train.so.8.0.5
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

Versions of relevant libraries:
[pip3] numpy==1.21.6
[pip3] torch==1.11.0+cu113
[pip3] torchaudio==0.11.0+cu113
[pip3] torchsummary==1.5.1
[pip3] torchtext==0.12.0
[pip3] torchvision==0.12.0+cu113
[conda] Could not collect

Start by running the linked Google Colab notebook with the reported PyTorch 1.11.0 and torchvision 0.12.0 environment. Then inspect coremltools/converters/mil/frontend/torch/converter.py around _lower_graph_block, where the KeyError for images.7 occurs. Done means the notebook converts the custom model to an .mlmodel without this RuntimeError.

Written by the indexing model from the issue text.