Updating the pylearn2 called classes for the parts that have changed.

I have modified example_convolution.yaml to update it with respect to the changes made recently to pylearn2. Note that if you run this yaml file you get the following error which is (based on what David W. told me) due to the recent updates of pylearn2. So, fix the following error:

Traceback (most recent call last):
  File "train.py", line 164, in <module>
    train_obj = serial.load_train_file(args.config)
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/utils/serial.py", line 529, in load_train_file
    return yaml_parse.load_path(config_file_path, environ=environ)
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/config/yaml_parse.py", line 97, in load_path
    return load(content, environ=environ, **kwargs)
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/config/yaml_parse.py", line 59, in load
    return instantiate_all(proxy_graph)
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/config/yaml_parse.py", line 151, in instantiate_all
    graph[key] = instantiate_all(graph[key])
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/config/yaml_parse.py", line 151, in instantiate_all
    graph[key] = instantiate_all(graph[key])
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/config/yaml_parse.py", line 163, in instantiate_all
    graph = graph.instantiate()
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/config/yaml_parse.py", line 228, in instantiate
    self.instance = checked_call(self.cls, self.kwds)
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/utils/call_check.py", line 98, in checked_call
    return to_call(**kwargs)
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/datasets/dense_design_matrix.py", line 1384, in from_dataset
    rval = DenseDesignMatrix(topo_view=V, y=y)
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/datasets/dense_design_matrix.py", line 173, in __init__
    self.set_topological_view(topo_view, axes)
  File "/data/lisatmp/honaris/pythonlibs/pylearn2/datasets/dense_design_matrix.py", line 751, in set_topological_view
    assert self.max_labels
AssertionError