zero-byte file in d2l-en.zip

[xiaotinghe]

Tensorflow and Torch implementations in the d2l-en.zip contain some 0-byte files

• chapter_convolutional-neural-networks/why-conv.ipynb
• chapter_convolutional-neural-networks/index.ipynb
• chapter_linear-networks/softmax-regression.ipynb
• chapter_linear-networks/index.ipynb
• ...

[AnirudhDagar]

I'll try to see what can be done here in d2lbook when I work on the new infra soon.