[pallas] Make the LoweringDynamicShapeEnv use local mappings
This fixes Pallas behavior in presence of shape polymorphism.
Previously, the LoweringDynamicShapeEnv was using global
mappings dim_expr_to_placeholder and placeholder_to_dim_expr.
This results in errors where we carry-over dim_expr from
one lowering to the next.
Concretely, this resulted in failures of the form
```
Encountered dimension variable 'b' that is not appearing in the shapes of the function arguments.
```
because the symbolic variable 'b' is carried over from a previous test
with Pallas and shape polymorphism.
