[RELAY] Move frontend utils (apache#5345)

* [RELAY] Move frontend utils

The util file currently under frontend is used from
outside of frontend (in qnn/op/legalizations). This suggests
that the file should be pushed up to a higher level.

The benefit from this change is that importing qnn no longer
also imports all the frontends.

* Inline get_scalar_from_constant

Change-Id: I1cc64e9ecb0eadb6ac0f7b62e6ea174644af4ad4

* Remove util.py from Relay

Change-Id: If9cd7cf3fc0bd1861a3a9b5604f338e084d8db96

* Shorten functions

Change-Id: Ieb537d82e6ee52421ff05a90cd00a03679ffebf2

* Line length

Change-Id: I1d216b7e73a060c4f118f5da50ce58b18eba907f

python/tvm/relay/frontend/tflite.py

@@ -29,7 +29,6 @@
 from .. import op as _op
 from .. import qnn as _qnn
 from ... import nd as _nd
-from .util import get_scalar_from_constant
 from .common import ExprTable
 from .common import infer_shape as _infer_shape
 
@@ -2341,6 +2340,17 @@ def get_expr(self, input_tensor_idx):
     def has_expr(self, input_tensor_idx):
         return self.exp_tab.has_expr(get_tensor_name(self.subgraph, input_tensor_idx))
 
+
+def get_scalar_from_constant(expr):
+    """ Returns scalar value from Relay constant scalar. """
+    assert isinstance(expr, _expr.Constant) and not expr.data.shape, \
+        "Expr is not a constant scalar."
+    value = expr.data.asnumpy()
+    assert value.dtype == np.dtype(np.int32) or value.dtype == np.dtype(np.float32), \
+        "value must be float32/int32"
+    return np.asscalar(value)
+
+
 def build_str_map(obj):
     """Build string map of TFLite enum int value
 

python/tvm/relay/qnn/op/legalizations.py

@@ -20,8 +20,8 @@
 
 import tvm
 from tvm import relay
+import numpy as np
 from .. import op as reg
-from ...frontend.util import get_scalar_from_constant
 
 #################################################
 # Register the functions for different operators.
@@ -54,6 +54,15 @@ def qnn_dense_legalize(attrs, inputs, types):
 # Helper functions.
 ###################
 
+def get_scalar_from_constant(expr):
+    """ Returns scalar value from Relay constant scalar. """
+    assert isinstance(expr, relay.Constant) and not expr.data.shape, \
+        "Expr is not a constant scalar."
+    value = expr.data.asnumpy()
+    assert value.dtype == np.dtype(np.int32) or value.dtype == np.dtype(np.float32), \
+        "value must be float32/int32"
+    return np.asscalar(value)
+
 # Helper function for lowering in the abscence of fast Int8 arithmetic units.
 def helper_no_fast_int8_hw_legalization(attrs, inputs, types, relay_op):
     """ Converts QNN operators into a sequence of Relay operators that are friendly to HW that do