`model.c` doesn't provide definition for `ParameterCollection(float weight_decay_lambda);`

In the header file `model.h`, there is a method declaration:
```c
ParameterCollection(float weight_decay_lambda);
```
But it seems that it has no corresponding implementation in `model.c`.
Instead,
```c
ParameterCollection::ParameterCollection(const string & my_name, ParameterCollection* my_parent, float weight_decay_lambda) :
name(my_name), storage(new ParameterCollectionStorage(weight_decay_lambda)), parent(my_parent) { }
```
is provided.

Inspect the ParameterCollection declarations in model.h and the existing definitions in model.c, starting with the constructor signatures shown in the issue. Check how the weight_decay_lambda value is expected to be handled, then verify that the declaration and implementation agree and that the project builds and its relevant tests pass.

Written by the indexing model from the issue text.