[rewriter] Unify reshape flatten ops (#2518)

Following (#2301),
`flatten_to_reshape_rule` rule set is introduced to reduce the following
list of operators:
- Reshape ∘ Flatten  -> Reshape
- Flatten ∘ Reshape  -> Reshape

Note to support this changes: 
- `ReshapeReshape` rule is updated to support more cases.
- `Flatten2Reshape` rule is introduced to convert Flatten ops into
Reshape when possible.

Author: Johansmm

onnxscript/rewriter/rules/common/__init__.py

@@ -10,6 +10,7 @@
     "div_by_1_rule",
     "dropout_inference_rule",
     "dropout_zero_rule",
+    "flatten_to_reshape_rule",
     "fuse_batchnorm_into_conv_rule",
     "fuse_batchnorm_into_conv_transpose_rule",
     "fuse_batchnorm_into_gemm_rule",
@@ -48,6 +49,7 @@
 
 from onnxscript.rewriter.rules.common._basic_rules import (
     cast_cast_rule,
+    flatten_to_reshape_rule,
     no_op_cast_rule,
     no_op_expand_rule,
     no_op_transpose_rule,

onnxscript/rewriter/rules/common/_basic_rules.py

@@ -11,6 +11,8 @@
 
 from typing import ClassVar, Sequence
 
+import numpy as np
+
 from onnxscript import ir
 from onnxscript.rewriter import _ir_utils as ir_utils
 from onnxscript.rewriter._basics import MatchResult
@@ -123,16 +125,37 @@ def pattern(self, op, x, shape_ignored, shape):
         return op.Reshape(op.Reshape(x, shape_ignored), shape)
 
     def rewrite(self, op, x: ir.Value, shape_ignored: ir.Value, shape: ir.Value):
-        return op.Reshape(x, shape)
+        new_shape = op.initializer(ir.Tensor(self._new_shape, name=shape.name))
+        return op.Reshape(x, new_shape, allowzero=self._allowzero)
 
     def check(self, context, x, shape_ignored, shape) -> MatchResult:
         check_result = MatchResult()
-        if shape_ignored.const_value is None:
-            return check_result.fail("Shape ignored is not a constant.")
-        if shape.const_value is None:
+
+        # Shape must be a constant.
+        if (np_shape := ir_utils.get_numpy_value(shape)) is None:
             return check_result.fail("Shape is not a constant.")
-        if shape.const_value.numpy().min() <= 0:
-            return check_result.fail("Shape has non-positive values.")
+        # Convert to array to support assignment destination.
+        self._new_shape = np.array(np_shape, np_shape.dtype)
+
+        # Try to replace {0,-1} values in shape if reshape output is known.
+        if (reshape_output := context.output_values[0].shape) is not None:
+            for i, dim in enumerate(reshape_output):
+                if isinstance(dim, int) and dim > 0:
+                    self._new_shape[i] = dim
+
+        # Constraints for shape.
+        self._allowzero = context.nodes[0].attributes.get_int("allowzero", 0)
+        if self._allowzero == 1 and any(self._new_shape == 0):
+            return check_result
+        if any(self._new_shape == 0) and any(self._new_shape < 0):
+            return check_result.fail("Shape cannot contain both 0 and -1 dimensions.")
+        elif np.count_nonzero(self._new_shape == 0) > 1:
+            return check_result.fail("Shape cannot contain more than one 0 dimension.")
+
+        # At this point, we can safely replace '0' with '-1'.
+        # Note allowzero is removed since at this point it does not have any effect.
+        self._allowzero = None
+        self._new_shape = np.where(self._new_shape == 0, -1, self._new_shape)
         return check_result
 
 
@@ -279,6 +302,55 @@ def check(self, context, x, axes1, axes2) -> MatchResult:
         return check_result
 
 
+class Flatten2Reshape(RewriteRuleClassBase):
+    """Convert ``Flatten(x)`` to Reshape."""
+
+    def pattern(self, op, x: ir.Value):
+        return op.Flatten(x)
+
+    def rewrite(self, op, x: ir.Value):
+        new_shape = op.initializer(ir.Tensor(self._new_shape, name=f"{x.name}/shape"))
+        return op.Reshape(x, new_shape)
+
+    def check(self, context, x: ir.Value) -> MatchResult:
+        check_result = MatchResult()
+        self._new_shape = np.array([-1, -1], "int64")
+
+        # Convert axis in a positive value if possible.
+        axis = context.root.attributes.get_int("axis", 1)
+        input_rank = None
+        if (input_shape := x.shape) is not None:
+            input_rank = len(input_shape)
+            if axis < 0:
+                axis += input_rank
+
+        # Compute reshape shape following axis attribute.
+        if axis == 0:
+            self._new_shape[0] = 1
+        elif axis == 1:
+            self._new_shape[0] = 0
+        elif axis == input_rank:
+            self._new_shape[1] = 1
+
+        # Try to update shape if output is known.
+        if (output_shape := context.output_values[0].shape) is not None:
+            for i, dim in enumerate(output_shape):
+                if isinstance(dim, int):
+                    self._new_shape[i] = dim
+
+        # Try to update shape if input is known.
+        if input_shape is not None:
+            if all(isinstance(dim, int) for dim in input_shape[:axis]):
+                self._new_shape[0] = np.prod(input_shape[:axis])
+            if all(isinstance(dim, int) for dim in input_shape[axis:]):
+                self._new_shape[1] = np.prod(input_shape[axis:])
+
+        # Verify if it is possible to apply rule.
+        if np.count_nonzero(self._new_shape == -1) > 1:
+            return check_result.fail("Impossible to compute new shape.")
+        return check_result
+
+
 # Create rule instances
 cast_cast_rule = CastCast.rule()
 no_op_cast_rule = CastIdentity.rule()
@@ -289,6 +361,7 @@ def check(self, context, x, axes1, axes2) -> MatchResult:
 transpose_transpose_rule = TransposeTranspose.rule()
 unsqueeze_unsqueeze_rule = UnsqueezeUnsqueeze.rule()
 squeeze_reshape_1d_rule = SqueezeReshape.rule()
+flatten_to_reshape_rule = Flatten2Reshape.rule()
 
 
 def basic_optimization_rules() -> RewriteRuleSet:
@@ -311,6 +384,8 @@ def basic_optimization_rules() -> RewriteRuleSet:
             cast_cast_rule,
             no_op_cast_rule,
             no_op_expand_rule,
+            # flatten_to_reshape_rule is order sensitive to reshape_reshape_rule
+            flatten_to_reshape_rule,
             reshape_reshape_rule,
             slice_split_rule,
             no_op_transpose_rule,