Fix type mismatch bug and remove dead code in IndexRowwiseMinMax::sa_decode_impl (#5051)

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/5051

In `sa_decode_impl<StorageMinMaxT>()`, a local variable `std::vector<StorageMinMaxFP16> minmax(...)` was:

1. Using the wrong type: It hardcoded `StorageMinMaxFP16` instead of the template parameter `StorageMinMaxT`. When this function is instantiated with `StorageMinMaxFP32` (via `IndexRowwiseMinMax::sa_decode`), this would create a vector of the wrong type.

2. Dead code: The vector was allocated but never actually used in the function body. The decoding logic reads `StorageMinMaxT` values directly from the input byte buffer, making this allocation unnecessary.

This change removes the unused variable, eliminating both the type mismatch and the unnecessary memory allocation. The allocation was O(min(chunk_size, n)) per decode call, so removing it also provides a minor performance benefit.

Note: The corresponding `sa_encode_impl` function does NOT have this issue (it correctly uses a local `minmax` that IS used), and the `train_inplace_impl` / `train_impl` functions also correctly use their `minmax` vectors. Only `sa_decode_impl` had this issue.

Reviewed By: mnorris11

Differential Revision: D99851973

fbshipit-source-id: a288a4cd355ccc7d9e13d1f7d61bc54fc524675c

faiss/IndexRowwiseMinMax.cpp

@@ -166,11 +166,6 @@ void sa_decode_impl(
                      ? chunk_size
                      : static_cast<size_t>(n_input)) *
             old_code_size);
-    std::vector<StorageMinMaxFP16> minmax(
-            (chunk_size < static_cast<size_t>(n_input)
-                     ? chunk_size
-                     : static_cast<size_t>(n_input)));
-
     // all the elements to process
     size_t n_left = n_input;