[functorch] Bump functorch main version to 0.2.0 (pytorch/functorch#448)

diff --git a/functorch/version.txt b/functorch/version.txt
index 8ea4f48..9e8f5f9 100644
--- a/functorch/version.txt
+++ b/functorch/version.txt
@@ -1 +1 @@
-0.0.1a0
+0.2.0a0