Update on "[flat_tensor] Persist FreeableBuffers of external constants in method"

## Problem
Currently, the FlatTensorDataMap persists tensors, and returns a FreeableBuffer with an empty free function.

The NamedDataMap should not persist data, as most cases (eg. delegate) will want it to be freed.

Ownership should be on the caller; `get_data` returns a FreeableBuffer that 'owns' the data. The FreeableBuffer in turn is owned by the caller.

NOTE: this doesn't support the case where we want to share plain tensors between methods/pte files at runtime. A custom NDM could support that use-case.

## This diff:
1. Introduces a 'NamedData' struct to method.h. This holds a key and a FreeeableBuffer.
2. Iterate over all the flatbuffer tensors to count the constants tagged with EXTERNAL.
NOTE: this will increase load time for all users. Potentially allocate chunks of 16 and use a linked list to store external constants, or store this number in PTE file.
3. Allocate space for num_external_constants using the method allocator.
4. Iterate over all flatbuffer tensors and use the named_data_map to resolve EXTERNAL tensors into the array of NamedData.
5. Pass the resolved external constants to tensor_parser, along with NDM (used for mutable external tensors).
6. Resolved external tensors are stored inside method. They are freed when the method is destructed.


Some notes:
https://docs.google.com/document/d/1_PBi4JgODuClUPD4PCUWrKNjyUH54zOUHGUJ3QHDNes/edit?tab=t.0#heading=h.blsvwraxss7g

Differential Revision: [D69477027](https://our.internmc.facebook.com/intern/diff/D69477027/)

[ghstack-poisoned]

Author: lucylq

runtime/executor/method.cpp

@@ -392,7 +392,7 @@ Error Method::parse_external_constants(const NamedDataMap* named_data_map) {
     new (&external_constants_[n_external_constants_].buffer)
         FreeableBuffer(std::move(buffer.get()));
 
-    n_external_constants_ = i + 1;
+    n_external_constants_ += 1;
   }
   return Error::Ok;
 }

runtime/executor/tensor_parser.h

@@ -35,7 +35,7 @@ ET_NODISCARD Result<executorch::aten::Tensor> parseTensor(
     MemoryManager* memory_manager,
     const executorch_flatbuffer::Tensor* s_tensor,
     const NamedDataMap* named_data_map = nullptr,
-    Span<NamedData> entries = {});
+    Span<NamedData> external_constants = {});
 
 ET_NODISCARD Result<BoxedEvalueList<executorch::aten::Tensor>> parseTensorList(
     const flatbuffers::Vector<int32_t>* tensor_indices,