[cortex_m] Fix linear weight layout: transpose in AOT pass, align meta/ref impl (pytorch#16782)

### Summary:
The linear path in ConvertToCortexMPass was not transposing weights
unlike
conv2d, causing inconsistency with the C++ runtime which expects weights
in
       [in_features, out_features] format per CMSIS-NN.
    
      Changes:
- convert_to_cortex_m_pass.py: Transpose linear weights [out, in] ->
[in, out]
- operators.py: Update meta to use weights.shape[1] for output dimension
- operators.py: Remove .T from ref impl (weights pre-transposed by pass)
    
      Fixes MV2 output shape mismatch: [1, 1280] -> [1, 1000]
    
      MV2 on Corstone-300/E8 with CMSIS-NN kernels
This fix ensures the AOT-compiled .pte file has correctly shaped output
tensors for any model using quantized_linear (MV2, ResNet, MV3, etc.).


### Test plan
Run MV2 lowered to CMSIS-NN ops on E8 alif board

Co-authored-by: Github Executorch <github_executorch@arm.com>

Author: psiddh

backends/cortex_m/ops/operators.py

@@ -352,7 +352,7 @@ def quantized_linear_meta(
     activation_min,
 ) -> torch.Tensor:
 
-    shape = (*input.shape[:-1], weights.shape[0])
+    shape = (*input.shape[:-1], weights.shape[1])
     return torch.empty(shape, dtype=input.dtype, device=input.device)
 
 
@@ -386,7 +386,7 @@ def quantized_linear_impl(
         input_reshaped = input_int32.reshape(new_shape)
 
         lhs_sum = torch.sum(input_reshaped, dim=-1, keepdim=True) * filter_offset
-        output = torch.mm(input_reshaped, weights_int32.T) + lhs_sum + kernel_sum
+        output = torch.mm(input_reshaped, weights_int32) + lhs_sum + kernel_sum
         output_shape = (*input.shape[:-1], output.shape[-1])
         output_reshaped = output.reshape(output_shape)
     else:
@@ -396,7 +396,7 @@ def quantized_linear_impl(
         new_shape = (prod(input.shape[:-1]), input.shape[-1])
         input_reshaped = input_int32.reshape(new_shape)
 
-        output = torch.mm(input_reshaped, weights_int32.T)
+        output = torch.mm(input_reshaped, weights_int32)
         if bias is not None:
             output = output + bias
         output_shape = (*input.shape[:-1], output.shape[-1])

backends/cortex_m/passes/convert_to_cortex_m_pass.py

@@ -33,14 +33,19 @@ class ConvertToCortexMPass(XNNPACKPass):
     by call_operator.
     """
 
-    def _compute_kernel_sum(self, weights, bias, input_offset, weight_offset):
+    def _compute_kernel_sum(
+        self, weights_transposed, bias, input_offset, weight_offset
+    ):
         """
         Computes the precomputed kernel sum term (bias optional)
             a * sum_j(wij + b) + ci
 
         for i = (1, ..., n), where j indexes the input activations.
+
+        Args:
+            weights_transposed: Weights already in [in_features, out_features] format
         """
-        weights_transposed = weights.T
+        # No transpose needed - weights already transposed by caller
         weights_int32 = weights_transposed.to(torch.int32)
         offset_weights = weights_int32 + weight_offset
         kernel_sum = torch.sum(offset_weights, dim=0, keepdim=True, dtype=torch.int32)
@@ -110,8 +115,12 @@ def _get_linear_replacement(self, node):
             if len(node.args) > 2
             else None
         )
+        # Transpose weights once from PyTorch format [out_features, in_features]
+        # to CMSIS-NN format [in_features, out_features]
+        weights_transposed = weights_tensor.T.contiguous()
+        # Pass already-transposed weights to kernel_sum computation
         kernel_sum_tensor = self._compute_kernel_sum(
-            weights_tensor, bias_tensor, -input_zp, -weight_zp
+            weights_transposed, bias_tensor, -input_zp, -weight_zp
         )
         with node.graph.inserting_after(weights):
             kernel_sum = create_constant_placeholder(
@@ -122,9 +131,17 @@ def _get_linear_replacement(self, node):
                 kernel_sum_tensor,
             )
 
+            weights_transposed_node = create_constant_placeholder(
+                self.exported_program,
+                node.graph,
+                node.name + "_weights_transposed",
+                InputKind.PARAMETER,
+                weights_transposed,
+            )
+
         args = (
             node.args[0],
-            weights,
+            weights_transposed_node,
             None,
             kernel_sum,
             -input_zp,