Move mediatek.passes to mediatek._passes (#5922)

Summary:
Changing mediatek.passes to mediatek._passes to indicate that these passes are not covered under the API stability guarantee.

Pull Request resolved: #5922

Reviewed By: helunwencser

Differential Revision: D63926846

fbshipit-source-id: fcea1a560892200f9ca0e110104908de0be25867

Author: tarun292

backends/mediatek/passes/__init__.py renamed to backends/mediatek/_passes/__init__.py

@@ -7,7 +7,7 @@
 from torch.ao.quantization.quantizer import Quantizer
 from torch.fx import GraphModule
 
-from ..passes.decompose_scaled_dot_product_attention import (
+from .._passes.decompose_scaled_dot_product_attention import (
     DecomposeScaledDotProductAttention,
 )
 from .annotator import annotate