optimize sampled_addmm performance on CPU (SparseCSR) (#90978)
### Target and Background
This PR is improving the performance of `sampled_addmm` on CPU device. This is part of effort for improving PyG performance on CPU for GNN training/inference.
The current implementation is a reference design which converts `SparseCSR` tensor back to dense tensor and then do the addmm and convert back to `SparseCSR` again: this is going to be very slow and won't be able to run most of the datasets under https://github.com/snap-stanford/ogb (convert to dense would trigger `OOM`).
### Benchmarks
Right now we don't have any hands-on benchmark or workload to test this since this operator is not used in PyG yet. I fetched the dataset from `ogb-products` where:
* number of nodes: 2.4 * 10^6
* number of edges: 1.26 * 10^8
* number of features: 128
So if we store the **adjacency matrix** is dense, it is going to be 2.4 * 2.4 * 4 * 10^12 bytes, this will be OOB on current code. I abstract the first 1k rows to compare, **1100x** speedup:
CPU: Intel(R) Xeon(R) Gold 6248 CPU @ 2.50GHz, dual socket, 20 cores per socket.
```
### before: run 1000 rows from the whole dataset
sampled_addmm: running dataset ogb-products first 1000 rows: each iter takes 1212.000 ms!
### after: run 1000 rows from the whole dataset
sampled_addmm: running dataset ogb-products first 1000 rows: each iter takes 1.102 ms!
### after: run the whole dataset
sampled_addmm: running dataset ogb-products (the whole dataset) 2449029 rows: each iter takes 873.306 ms!
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90978
Approved by: https://github.com/pearu, https://github.com/cpuhrsch
