[Lighting Module] Add half_lambert & blinn_phong

[tomguluson92]

Hi,
Since I found there is only a basic phong renderer for specular and lambert renderer for diffuse . I add two commonly used renderer: blinn_phong and half_lambert to strengthen the APIs.

With no intention of do damage to the consistency of current API, I add a little bit about interfaces in `lighting.py`, `shading.py` and `shader.py`. 

Plus, it will be a great honor for me if this helps, thanks a lot for your brilliant pytorch3D repo!

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[nikhilaravi]

@tomguluson92 thanks for making this PR and apologies for the late response! We will review the code and reply back with comments. Stay tuned!

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[tomguluson92]

@tomguluson92 thanks for making this PR and apologies for the late response! We will review the code and reply back with comments. Stay tuned!

Thanks ^.^

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