[NFC] Remove unused operation

[Mr-Anyone]

Motivation

Remove unused operations.

Technical Details

I suspect that this operation is never used for the following reasons:

There is no lowering from migraphx.flatten to the Tosa pipeline.

root@31dddf580291 ~/rocMLIR/build-release ((HEAD detached at origin/develop))$ ./bin/rocmlir-opt main.mlir  --migraphx-to-tosa
main.mlir:2:3: error: failed to legalize operation 'migraphx.flatten' that was explicitly marked illegal
  migraphx.flatten %arg0 {axis = 0 : i64}: <1x1xf32, 1x1> -> <1xf32, 1>
  ^
main.mlir:2:3: note: see current operation: %0 = "migraphx.flatten"(%arg0) <{axis = 0 : i64}> : (!migraphx.shaped<1x1xf32, 1x1>) -> !migraphx.shaped<1xf32, 1>
root@31dddf580291 ~/rocMLIR/build-release ((HEAD detached at origin/develop))$ 

There is no reference of FlattenOp in MIGraphXToTosa.cpp.

I ran git log --oneline | grep flatten and nothing significant seems to show up.

root@31dddf580291 ~/rocMLIR/build-release ((HEAD detached at origin/develop))$ git log --oneline | grep flatten
c74e8539ff37 [Analysis] flatten enums for recurrence types
d00f59de80f1 [test][NewPM] Pin -flattencfg test to legacy PM
d53b4bee0ccd [LoopFlatten] Add a loop-flattening pass
9e783716a224 [llvm-libtool-darwin] Allow flattening archives
55d5e6cbe250 [flang] Remove flatten and merge upstreaming script.
42cc44fbc8dc [flang] Add script to flatten git history for llvm monorepo submission (flang-compiler/f18#854)
5c5880db2f17 [flang] Merge pull request flang-compiler/f18#868 from schweitzpgi/remove-flatten
d98cb81cd11c Handle successor's PHI node correctly when flattening CFG merges two if-regions
957380714da3 [clangd] Unfold SourceLocation flattening from findNameLoc in preparation for adding more overloads. NFC
1693b80bd567 [SimplifyCFG][NFC] Test that we fail to flatten CFG in JPEG "sign" value extend pattern
fca23d74c963 [SimplifyCFG][NFC] Test that we fail to flatten CFG after forming @llvm.umul.with.overflow
ec66bc57a879 Add helper to get flattened tuple types

It was introduced in this commit - the inception of MIGraphX dialect and it doesn't have any testcase.

Test Plan

N/A

Test Result

N/A

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2306      +/-   ##
===========================================
- Coverage    79.50%   78.89%   -0.62%     
===========================================
  Files          100      123      +23     
  Lines        31016    40444    +9428     
  Branches      4819     6638    +1819     
===========================================
+ Hits         24659    31905    +7246     
- Misses        4245     5634    +1389     
- Partials      2112     2905     +793     

Flag	Coverage Δ
mfma	?
navi4x	78.89% <ø> (?)

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see 101 files with indirect coverage changes

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