fix algorithm of spreading vectors over shards (#3374)

Summary:
simple math:
| **input n** | **input nshards** |  shard_size | idx | i0 | ni |
| -- |-- |-- |-- |-- |-- |
| 19 | 6 | 4 | 5 | 20 | **-1** |
| 1000 | 37 | 28 | 36 | 1008 | -8 |
| 1000 | 64 | 16 | 63 | 1008 | -8 |

root cause:
integer cause precision loss, `idx * shard_size` overflows, because `(n + nshards - 1) / nshards` is roundup

my solution:
each shard takes at least  `base_shard_size = n / nshards`, then `remain = n % nshards`, we know `0 <= remain < nshards`, next, assign those remain vectors to first `remain` shards, i.e. first `remain` shards take one more vector each.
```c++
auto i0 = idx * base_shard_size;
if (i0 < remain) {
  // if current idx is one of the first `remain` shards
  i0 += idx;
} else {
  i0 += remain;
}
```
simplify above code: `i0 = idx * base_shard_size + std::min(size_t(idx), n % nshards);`

Pull Request resolved: https://github.com/facebookresearch/faiss/pull/3374

Reviewed By: fxdawnn

Differential Revision: D57867910

Pulled By: junjieqi

fbshipit-source-id: 7e72ea5cd197af4f3446fb7a3fd34ad08901dbb2

faiss/gpu/GpuIcmEncoder.cu

@@ -82,7 +82,7 @@ void GpuIcmEncoder::encode(
         size_t n,
         size_t ils_iters) const {
     size_t nshards = shards->size();
-    size_t shard_size = (n + nshards - 1) / nshards;
+    size_t base_shard_size = n / nshards;
 
     auto codebooks = lsq->codebooks.data();
     auto M = lsq->M;
@@ -94,8 +94,14 @@ void GpuIcmEncoder::encode(
 
     // split input data
     auto fn = [=](int idx, IcmEncoderImpl* encoder) {
-        size_t i0 = idx * shard_size;
-        size_t ni = std::min(shard_size, n - i0);
+        size_t i0 = idx * base_shard_size + std::min(size_t(idx), n % nshards);
+        size_t ni = base_shard_size;
+        if (ni < n % nshards) {
+            ++ni;
+        }
+        if (ni <= 0) { // only if n < nshards
+            return;
+        }
         auto xi = x + i0 * d;
         auto ci = codes + i0 * M;
         std::mt19937 geni(idx + seed); // different seed for each shard