A possible memory leak when an exception is thrown

In file faiss/impl/index_read.cpp, function `read_VectorTransform(IOReader* f)`.

Those pointers allocated by `new` (`pca`, `rdt`, `nt`, etc.) might leak when an exception is thrown.

`READ1`, `READVECTOR` and those `FAISS_THROW_XXX` could all throw exceptions.

I think it'll be better to wrap those raw pointers by smart pointers.

Start in faiss/impl/index_read.cpp at read_VectorTransform(IOReader* f), and trace the pca, rdt, nt and other allocations alongside READ1, READVECTOR and the FAISS_THROW_XXX paths. Confirm that exceptions during each read do not leave allocated objects unreleased; done means the function is exception-safe without changing successful reads.

Written by the indexing model from the issue text.