[fix](variant) change session var for disable_variant_flatten_nested to enable_variant_flatten_nested

[amorynan]

What problem does this PR solve?

change session var for disable_variant_flatten_nested to enable_variant_flatten_nested
Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[amorynan]

run buildall

[doris-robot]

TPC-H: Total hot run time: 33459 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpch-tools
Tpch sf100 test result on commit 9f239698e97facc52786f2a7e74dcdebfc4f5c86, data reload: false

------ Round 1 ----------------------------------
q1	17586	5225	5137	5137
q2	1915	283	186	186
q3	10322	1333	731	731
q4	10210	982	516	516
q5	7543	2284	2353	2284
q6	197	164	132	132
q7	876	788	611	611
q8	9315	1189	1024	1024
q9	6785	4993	5028	4993
q10	6887	2332	1967	1967
q11	457	295	267	267
q12	355	358	227	227
q13	17765	3409	2949	2949
q14	234	245	214	214
q15	522	469	473	469
q16	428	426	365	365
q17	580	801	368	368
q18	7365	7126	6966	6966
q19	909	987	559	559
q20	320	318	226	226
q21	3361	3123	2288	2288
q22	1044	1009	980	980
Total cold run time: 104976 ms
Total hot run time: 33459 ms

----- Round 2, with runtime_filter_mode=off -----
q1	5360	5373	5329	5329
q2	236	304	224	224
q3	2066	2558	2210	2210
q4	1328	1731	1279	1279
q5	4156	4204	4483	4204
q6	225	180	135	135
q7	1994	1916	1810	1810
q8	2577	2599	2497	2497
q9	7314	7474	7122	7122
q10	3045	3261	2923	2923
q11	535	532	501	501
q12	834	990	724	724
q13	3491	3718	3228	3228
q14	310	315	294	294
q15	495	460	446	446
q16	456	469	479	469
q17	1182	1508	1414	1414
q18	14887	7948	7830	7830
q19	14079	933	904	904
q20	2309	1997	1885	1885
q21	15920	4397	4290	4290
q22	1072	1051	1018	1018
Total cold run time: 83871 ms
Total hot run time: 50736 ms

[doris-robot]

TPC-DS: Total hot run time: 170243 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpcds-tools
TPC-DS sf100 test result on commit 9f239698e97facc52786f2a7e74dcdebfc4f5c86, data reload: false

============================================
query1	996	361	392	361
query2	6503	1701	1704	1701
query3	6744	219	235	219
query4	26899	23977	22877	22877
query5	4355	658	542	542
query6	325	272	232	232
query7	4630	532	301	301
query8	293	256	224	224
query9	8591	3004	3020	3004
query10	466	332	304	304
query11	15485	15056	14758	14758
query12	182	138	134	134
query13	1676	557	424	424
query14	8549	5871	5855	5855
query15	218	192	164	164
query16	7776	672	457	457
query17	1641	780	652	652
query18	2070	449	329	329
query19	295	218	193	193
query20	157	149	144	144
query21	223	128	114	114
query22	4124	4082	4015	4015
query23	34569	34339	34173	34173
query24	8302	2417	2470	2417
query25	567	514	443	443
query26	922	290	163	163
query27	2628	507	367	367
query28	4251	2352	2352	2352
query29	703	594	499	499
query30	302	237	195	195
query31	881	813	752	752
query32	90	78	84	78
query33	566	410	366	366
query34	820	865	527	527
query35	800	879	778	778
query36	1028	1058	903	903
query37	133	123	93	93
query38	3935	3984	3901	3901
query39	1445	1412	1371	1371
query40	252	148	130	130
query41	67	60	55	55
query42	145	126	129	126
query43	525	528	499	499
query44	1477	891	896	891
query45	195	186	179	179
query46	967	1073	694	694
query47	1797	1846	1760	1760
query48	403	449	329	329
query49	705	508	417	417
query50	659	695	434	434
query51	4188	4197	4198	4197
query52	132	139	127	127
query53	264	312	236	236
query54	686	662	578	578
query55	90	90	87	87
query56	355	360	351	351
query57	1221	1241	1135	1135
query58	334	340	332	332
query59	2597	2618	2538	2538
query60	422	380	412	380
query61	125	125	127	125
query62	826	745	656	656
query63	261	224	225	224
query64	3538	1101	807	807
query65	4189	4143	4169	4143
query66	908	452	335	335
query67	query68	16947	636	613	613
query69	1000	324	310	310
query70	1456	1100	1118	1100
query71	692	346	322	322
query72	9122	2355	2417	2355
query73	3366	663	357	357
query74	9116	8958	8823	8823
query75	7375	3129	2609	2609
query76	8638	1271	821	821
query77	1150	429	341	341
query78	9580	10979	9347	9347
query79	15333	606	579	579
query80	2935	570	483	483
query81	548	266	236	236
query82	514	148	118	118
query83	399	291	322	291
query84	296	107	79	79
query85	914	375	340	340
query86	353	318	289	289
query87	4198	4127	4071	4071
query88	5486	2236	2244	2236
query89	478	374	325	325
query90	2488	234	229	229
query91	204	144	113	113
query92	90	72	70	70
query93	6818	991	654	654
query94	1412	405	292	292
query95	431	321	321	321
query96	514	595	289	289
query97	2678	2749	2591	2591
query98	239	226	221	221
query99	1486	1360	1271	1271
Total cold run time: 307724 ms
Total hot run time: 170243 ms

[doris-robot]

ClickBench: Total hot run time: 32.53 s

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/clickbench-tools
ClickBench test result on commit 9f239698e97facc52786f2a7e74dcdebfc4f5c86, data reload: false

query1	0.04	0.04	0.04
query2	0.08	0.04	0.04
query3	0.25	0.07	0.06
query4	1.62	0.10	0.10
query5	0.42	0.42	0.43
query6	1.16	0.66	0.66
query7	0.03	0.02	0.02
query8	0.05	0.04	0.04
query9	0.57	0.48	0.47
query10	0.52	0.52	0.52
query11	0.16	0.11	0.10
query12	0.15	0.12	0.11
query13	0.66	0.62	0.64
query14	0.90	1.10	1.05
query15	0.99	0.86	0.89
query16	0.39	0.40	0.41
query17	1.09	1.10	1.07
query18	0.21	0.20	0.20
query19	1.90	1.98	1.79
query20	0.02	0.01	0.02
query21	15.42	0.83	0.54
query22	0.76	1.12	0.72
query23	14.91	1.23	0.64
query24	6.71	1.96	0.32
query25	0.31	0.19	0.09
query26	0.58	0.17	0.14
query27	0.05	0.06	0.05
query28	9.35	0.87	0.43
query29	12.58	3.83	3.27
query30	3.12	3.06	2.97
query31	2.82	0.57	0.39
query32	3.24	0.57	0.49
query33	2.99	3.20	3.16
query34	16.16	5.33	4.93
query35	4.93	4.94	4.94
query36	0.72	0.52	0.51
query37	0.10	0.08	0.07
query38	0.06	0.05	0.05
query39	0.04	0.03	0.03
query40	0.19	0.13	0.13
query41	0.08	0.03	0.02
query42	0.04	0.03	0.03
query43	0.04	0.03	0.04
Total cold run time: 106.41 s
Total hot run time: 32.53 s

[amorynan]

run p0

[amorynan]

run cloud_p0

[eldenmoon]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[csun5285]

LGTM