Use FlatTensorHeader::kHeaderOffset for the FlatTensor size check (pytorch#22137)

### Summary

`FlatTensorDataMap::load` checks that the buffer is at least as long as
the flatbuffer header before it reads the root table offset out of it.
It computed that length as `sizeof(flatbuffers::uoffset_t) +
flatbuffers::kFileIdentifierLength`.

`kFileIdentifierLength` only exists at namespace scope in newer
flatbuffers releases. In flatbuffers 1.12 the same constant exists only
as a member of `FlatBufferBuilder`, so a build against an older
flatbuffers fails to compile:

```
flat_tensor_data_map.cpp:303:40: error: no member named 'kFileIdentifierLength' in namespace 'flatbuffers'
```

The submodule pinned in this repository is v24.3.25, where the namespace
scope name does exist, so CI here cannot see the problem.

### What this changes

The number being computed is 8: a 4 byte root table offset followed by a
4 byte file identifier. `FlatTensorHeader::kHeaderOffset` already names
exactly that number, because the FlatTensor header begins right after
the flatbuffer header ends. The matching bounds check in `Program::load`
already uses `ExtendedHeader::kHeaderOffset` for the same reason.

So this swaps the expression for `FlatTensorHeader::kHeaderOffset`. The
bound does not move, behavior is unchanged, and no include is added,
`flat_tensor_header.h` was already included by this file.

### Test plan

Compiled `extension/flat_tensor/flat_tensor_data_map.cpp` with
`-std=c++17 -Wall` against the pinned flatbuffers v24.3.25. Clean, no
warnings and no errors.

Checked that the fix is load bearing by compiling both spellings against
both flatbuffers versions:

| | old expression | new expression |
| --- | --- | --- |
| flatbuffers 1.12.0 | fails with the error above | compiles |
| flatbuffers 24.3.25 | compiles | compiles |

The probe also `static_assert`s that the new expression equals the old
one and equals 8, so the two spellings really do produce the same bound.

Author: shoumikhin

extension/flat_tensor/flat_tensor_data_map.cpp

@@ -296,11 +296,11 @@ ET_NODISCARD Result<const char*> FlatTensorDataMap::get_key(
   // so a corrupt offset would otherwise let version()/named_data()/segments()
   // dereference memory outside the buffer.
   //
-  // Minimum size: root offset (uoffset_t) + file identifier, i.e. the
-  // flatbuffer header. The identifier check above already implies at least this
-  // many bytes, but check explicitly so the bound below cannot underflow.
-  constexpr size_t kMinBufferSize =
-      sizeof(flatbuffers::uoffset_t) + flatbuffers::kFileIdentifierLength;
+  // Minimum size: root offset + file identifier, i.e. the flatbuffer header
+  // before the FlatTensor header begins. The identifier check above already
+  // implies at least this many bytes, but check explicitly so the bound below
+  // cannot underflow.
+  constexpr size_t kMinBufferSize = FlatTensorHeader::kHeaderOffset;
   ET_CHECK_OR_RETURN_ERROR(
       flat_tensor_data->size() >= kMinBufferSize,
       InvalidExternalData,