feat: add config for compiler mode (#1449)

exla/lib/exla.ex

@@ -71,6 +71,17 @@ defmodule EXLA do
       * `:highest` - Slowest but most accurate. Performs computations in float32
         or float64 as applicable
 
+    * `:compiler_mode` - the mode to use for the compiler. It must be one of:
+
+      * `:mlir` - The default mode. Uses MLIR to compile the computation
+
+      * `:xla` - Legacy implementation. Uses XLA to compile the computation
+
+    The `:compiler_mode` can also be set globally through a specific fallback config:
+
+        config :exla, :compiler_mode, :mlir
+
+
   ## Clients
 
   The `EXLA` library uses a client for compiling and executing code.

exla/lib/exla/defn.ex

@@ -581,7 +581,7 @@ defmodule EXLA.Defn do
 
           inputs_and_shapes = Enum.reverse(reverse_inputs_and_shapes)
 
-          mode = options[:compiler_mode] || :xla
+          mode = options[:compiler_mode] || Application.get_env(:exla, :compiler_mode, :mlir)
 
           {mod, builder} =
             case mode do