[mlir][OpDSL] Add default value to index attributes.

Index attributes had no default value, which means the attribute values had to be set on the operation. This revision adds a default parameter to `IndexAttrDef`. After the change, every index attribute has to define a default value. For example, we may define the following strides attribute:
```

```
When using the operation the default stride is used if the strides attribute is not set. The mechanism is implemented using `DefaultValuedAttr`.

Additionally, the revision uses the naming index attribute instead of attribute more consistently, which is a preparation for follow up revisions that will introduce function attributes.

Depends On D119125

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D119126

mlir/docs/Dialects/Linalg/OpDSL.md

@@ -105,7 +105,7 @@ appear in the parameter list of the operation:
 copy_and_scale(val, in_tensor, outs=[out_tensor])
 ```
 
-## Attributes
+## Index Attributes
 
 Attributes are compile-time constant parameters only accessible in index
 expressions. They can be used to parameterize the access pattern of a structured
@@ -118,7 +118,7 @@ The following example demonstrates the use of attributes:
 @linalg_structured_op
 def strided_copy(I=TensorDef(T, S.IH, S.IW),
                  O=TensorDef(T, S.OH, S.OW, output=True),
-                 strides=IndexAttrDef(S.SH, S.SW)):
+                 strides=IndexAttrDef(S.SH, S.SW, default=[1, 1])):
   """Copy a subset of the input tensor elements to the output tensor"""
   O[D.oh, D.ow] = I[D.oh * S.SH, D.ow * S.SW]
 ```
@@ -129,11 +129,12 @@ the symbols `S.SH` and `S.SW`, which are used to index the input tensor `I`.
 When instantiating the operation, the attribute is set using a named argument:
 
 ```python
-strided_copy(in_tensor, outs=[out_tensor], strides=[1,2])
+strided_copy(in_tensor, outs=[out_tensor], strides=[1, 2])
 ```
 
 The `strides` vector elements substitute the symbols `S.SH` and `S.SW` in the
-index expressions of the operation instance.
+index expressions of the operation instance. If no strides are provided the
+`default` vector elements are used instead.
 
 Attributes are currently limited to integer vectors and only accessible in index
 expressions. An operation may have multiple attributes all of them placed at the
@@ -157,8 +158,8 @@ def pooling_poly(
     I=TensorDef(T1, S.N, S.H, S.W, S.C),
     K=TensorDef(T2, S.KH, S.KW, index_dims=[D.kh, D.kw]),
     O=TensorDef(U, S.N, S.OH, S.OW, S.C, output=True),
-    strides=IndexAttrDef(S.SH, S.SW),
-    dilations=IndexAttrDef(S.DH, S.DW)):
+    strides=IndexAttrDef(S.SH, S.SW, default=[1, 1]),
+    dilations=IndexAttrDef(S.DH, S.DW, default=[1, 1])):
   O[D.n, D.oh, D.ow, D.c] += TypeFn.cast(U,
           I[D.n, D.oh * S.SH + D.kh * S.DH, D.ow * S.SW + D.kw * S.DW, D.c])
 ```