TypeError: can't convert expression to float when converting yolo.h5

## 🐞Describe the bug
When trying to convert a yolo.h5 model I get the error:
>TypeError: can't convert expression to float can't convert expression to float

Model: [Yolo Model](https://github.com/OlafenwaMoses/ImageAI/releases/download/1.0/yolo.h5)
Code: [Colab Notebook](https://colab.research.google.com/drive/1PEL2_ten0Fr9Tcem00rVNZIIJ15DOi68?usp=sharing)

## Trace

## To Reproduce
Check the provided notebook above.

## System environment (please complete the following information):
- coremltools version (e.g., 4.0b3):
- colab

Start by running the provided Colab notebook with the linked yolo.h5 model and inspect the conversion trace around the reported TypeError. Identify the conversion step that attempts to convert an expression to a float; done means the model converts successfully without that error.

Written by the indexing model from the issue text.