Adding [[fallthrough]] annotation to case statements in faiss/utils/h…

…amming_distance/generic-inl.h

Summary:
## Context
Due to a recent change (D52393687) to increase compiler warnings arcoss faiss/PACKAGE aggregator [build](https://www.internalfb.com/chronos/job/gp/9007207455707765) is failing. The annotation [[fallthrough]]; needs to be used in every [nested]case statement where fallthrough can occur.

## COMPILER ERROR
```
[2023-12-22T22:06:48.478-08:00] Action failed: fbcode//faiss:faiss (ovr_config//platform/linux:x86_64-fbcode-platform010-clang15-jemalloc-master#a916d57931e3a679) (cxx_compile impl/LocalSearchQuantizer.cpp)
[CONTEXT] [2023-12-22T22:06:48.478-08:00] Remote command returned non-zero exit code 1
[CONTEXT] [2023-12-22T22:06:48.478-08:00] Remote action, reproduce with: `frecli cas download-action d02e5a31e82ab2a6a2ce8b84dff4d4bce3997ca49783013f860d16dd566edaaf:145`
buck-out/v2/gen/fbcode/a916d57931e3a679/faiss/__faiss__/buck-headers/faiss/utils/hamming_distance/generic-inl.h:278:21: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
[CONTEXT]                     case 7:
[CONTEXT]                     ^
buck-out/v2/gen/fbcode/a916d57931e3a679/faiss/__faiss__/buck-headers/faiss/utils/hamming_distance/generic-inl.h:281:21: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
[CONTEXT]                     case 6:
[CONTEXT]                     ^
buck-out/v2/gen/fbcode/a916d57931e3a679/faiss/__faiss__/buck-headers/faiss/utils/hamming_distance/generic-inl.h:284:21: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
[CONTEXT]                     case 5:
```

## WHAT I DID
 +1 annotation **[[fallthrough]];** at every case statement that required it.

## WHAT I DIDNT DO
 - I did NOT **EXPORTED TO OSS FAISS**
   - mdouze please do this if it is required. I do not know your policy. Prior to landing i saw the message but was unsure. As well as not having github account.

Differential Revision: D52396801

fbshipit-source-id: a563068af00135f7fd24de6633007a1957fc6e31

faiss/utils/hamming_distance/generic-inl.h

@@ -275,24 +275,31 @@ struct HammingComputerDefault {
                     len -= 8;
                     accu += popcount64(a64[i] ^ b64[i]);
                     i++;
+                    [[fallthrough]];
                     case 7:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 6:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 5:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 4:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 3:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 2:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 1:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
@@ -302,20 +309,28 @@ struct HammingComputerDefault {
             const uint8_t* a = a8 + 8 * quotient8;
             const uint8_t* b = b8 + 8 * quotient8;
             switch (remainder8) {
+                [[fallthrough]];
                 case 7:
                     accu += hamdis_tab_ham_bytes[a[6] ^ b[6]];
+                    [[fallthrough]];
                 case 6:
                     accu += hamdis_tab_ham_bytes[a[5] ^ b[5]];
+                    [[fallthrough]];
                 case 5:
                     accu += hamdis_tab_ham_bytes[a[4] ^ b[4]];
+                    [[fallthrough]];
                 case 4:
                     accu += hamdis_tab_ham_bytes[a[3] ^ b[3]];
+                    [[fallthrough]];
                 case 3:
                     accu += hamdis_tab_ham_bytes[a[2] ^ b[2]];
+                    [[fallthrough]];
                 case 2:
                     accu += hamdis_tab_ham_bytes[a[1] ^ b[1]];
+                    [[fallthrough]];
                 case 1:
                     accu += hamdis_tab_ham_bytes[a[0] ^ b[0]];
+                    [[fallthrough]];
                 default:
                     break;
             }