add python intro & "what you should have learned sections"

pre-exercises/0_python.ipynb

pre-exercises/1_numpy.ipynb

pre-exercises/2_plotting.ipynb

pre-exercises/3_pandas.ipynb

pre-exercises/4_1_pytorch_tensors_autograd.ipynb

pre-exercises/4_2_pytorch_build_model.ipynb

pre-exercises/4_3_pytorch_model_optimization.ipynb

pre-exercises/5_wandb.ipynb

pre-exercises/helpers.py