Lightgcn fix

[theisjendal]

Description

LightGCN is a simplified version of NGCF, however, some of the simplifications made in LightGCN where not included.
Therefore:

• I have removed normalization between layers as this is not used by LightGCN.
• Fixed the sum constant used for adding layers to 1/(L+1) and instead of 1/(l+1) where L is the number of layers and l is the current layer index.
• Previous version of LightGCN didn't support blocks. It now supports DGL blocks.
• Fixed requirements versions as newer version of DGL/Torch created an error.
• I moved edge normalization to graph creation making computations in LightGCN easier/simpler.

Checklist:

• I have added tests.
• I have updated the documentation accordingly.
• I have updated README.md (if you are adding a new model).
• I have updated examples/README.md (if you are adding a new example).
• I have updated datasets/README.md (if you are adding a new dataset).

[theisjendal]

@tqtg as per #600 (comment), I've created a pull request for LightGCN only.

[qtuantruong]

thanks @theisjendal for clarifying all the changes. They all make sense to me after revising the authors' implementation of LightGCN. Could you also try to compare the updated version and the old one using this example with the same hyper-parameters? I'm curious about the changes in accuracy and speed of convergence.

[theisjendal]

@tqtg, here are the results:
OLD
Training: 11%, 110/1000 [13:45<1:50:48, 7.47s/iter, loss=0.00479]
Early stopping:

• best epoch = 61, stopped epoch = 111
• best monitored value = 0.247716 (delta = -0.005755)

NEW
Training: 62%, 619/1000 [1:11:29<44:35, 7.02s/iter, loss=0.00469]
Early stopping:

• best epoch = 570, stopped epoch = 620
• best monitored value = 0.244275 (delta = -0.001430)

VALIDATION:

            | NDCG@20 | Recall@20 | Time (s)
------------ + ------- + --------- + --------
LightGCN-Old |  0.1534 |    0.2420 |   4.9250
LightGCN-New |  0.1552 |    0.2428 |   4.8821

TEST:

             | NDCG@20 | Recall@20 | Train (s) | Test (s)
------------ + ------- + --------- + --------- + --------
LightGCN-Old |  0.1698 |    0.2503 |  831.2188 |   5.2217
LightGCN-New |  0.1717 |    0.2581 | 4294.9170 |   5.2097

[qtuantruong]

LGTM. Feel free to merge this PR.