Set test seed per-test (#19744)

### Summary
In #19651, I added a global
seed for pytest runs. This was intended to reduce random tolerance
flakes, but didn't actually do so in practice. This is because the
parallel test runners don't guarantee any ordering, so random state is
unstable between runs.

I've updated it to set the seed per-test. This should hopefully make the
random state invariant of test execution order.

Author: GregoryComer

backends/cadence/aot/tests/test_replace_ops_passes.py

@@ -1250,6 +1250,7 @@ def test_replace_conv1d_with_linear(self) -> None:
             inputs,
             "ReplaceTrivialConvWithLinear",
             rtol=2e-5,
+            atol=5e-6,
         )
 
         # Assert that conv1d is trivially converted to linear
@@ -1294,6 +1295,7 @@ def test_replace_conv2d_with_linear(self) -> None:
             inputs,
             "ReplaceTrivialConvWithLinear",
             rtol=2e-5,
+            atol=5e-6,
         )
 
         # Assert that conv2d is trivially converted to linear

conftest.py

@@ -1,3 +1,4 @@
+import hashlib
 import sys
 
 import torch
@@ -13,5 +14,8 @@
         "backends/apple/**",
     ]
 
-# Seed the run
-torch.manual_seed(42)
+
+def pytest_runtest_setup(item):
+    # Set a stable seed for each test based on a hash of the test name.
+    seed = int(hashlib.sha256(item.nodeid.encode()).hexdigest(), 16) % (2**32)
+    torch.manual_seed(seed)