Add support for traced if statements in onnxscript script (#2644)

Extend the onnxscript converter to handle conditionals that are
evaluated at script-time (that is, in the style of trace-mode). This
makes it easier to define parametric scripts that can be used to
generate variations of a pattern: for example, like the many variations
of the [SDPA pattern test
cases](https://github.com/microsoft/onnxscript/blob/8a94ad646440f462dd9ae1de6b303fe5f7b7f564/onnxscript/rewriter/ort_fusions/sdpa_test.py#L40).

This supports just a very basic version, where the if-condition is an
outer-scope variable, like below:
```py
   if outer_scope_variable:
      ...
   else:
      ...
```
For such cases, the script will just include the then or else branch as
appropriate, without generating an if-node.

Also: introduce an analyzer class to encapsulate analysis information,
and avoid updates to AST node.

TODO: some simple extension may be useful (perhaps allow any expression
in the if-condition that does not contain local variables).

---------

Signed-off-by: Ganesan Ramalingam <[email protected]>
Co-authored-by: Copilot <[email protected]>

Author: gramalingam