Tests: Added tests for Attributors

[chr5tphr]

Tests: Added tests for Attributors

Fixtures:

• add fixture for common grad_output functions, with an extra function
  one_hot_max
• add fixture for a simple model, which uses any of the linear module
  fixture, followed by a relu, followed by a dense layer with 4 outputs
• add a fixture for the simple model's gradient
• add a fixture for the simple model's output

Helpers:

• add the IdentityLogger helper in test_attribution.py, which is a
  torch.nn.Module which clones, logs and returns the input

Tests:

• add various tests for all attributors, which currently are Gradient,
  IntegratedGradients, SmoothGrad and Occlusion

Tests: Move composite fixtures to conftest

• move composite fixtures to conftest such that they can be used by
  other tests, and to conform to the 'fixtures in conftest.py' structure
• rename fixture 'composite_model' to 'model_vision'
• add alexnet to the model_vision fixture