Fix corner cases global shift gen

In some corner cases, the global shift factor was generated as a
number < 0 (down to -inf...). This makes no sense, so now the global
shift must be 0 at a minimum.

Author: FrancescoConti

test/NnxTestClasses.py

@@ -213,7 +213,11 @@ def _calculate_global_shift(
         """Calculate global shift so that the output values are in the range of out_type"""
         s = tensor.type(torch.float64).std()
         target_s = 2 ** (out_type._bits - 1)
-        return torch.ceil(torch.log2(s / target_s)).type(torch.int32)
+        shift = torch.ceil(torch.log2(s / target_s)).type(torch.int32)
+        if shift < 1:
+            return torch.zeros((1,)).type(torch.int32)
+        else:
+            return shift
 
     @staticmethod
     def _random_data(_type: IntegerType, shape: Tuple, extremes: Tuple = None):