Remove sparse test collection warning

[msaroufim]

Renamed the model from TestModel to ToyModel this is because pytest will by default try to collect any classes that start with the name Test

So this removes this warning

  test/sparsity/test_fast_sparse_training.py:17
    /pytorch/ao/test/sparsity/test_fast_sparse_training.py:17: PytestCollectionWarning: cannot collect test class 'TestModel' because it has a __init__ constructor (from: test/sparsity/test_fast_sparse_training.py)
      class TestModel(nn.Module):

[pytorch-bot]

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